docs: fault-injection: fix non-working usage of negative values

Fault injection uses debugfs in a way that the provided values via sysfs
are interpreted as u64. Providing negative numbers results in an error:

/sys/kernel/debug/fail_function# echo -1 > times
sh: write error: Invalid argument

Update the docs and examples to use "printf %#x <val>" in these cases.
For "retval", reword the paragraph a little and fix a typo.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210603125841.27436-1-wsa+renesas@sang-engineering.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: Wolfram Sang

Documentation/fault-injection/fault-injection.rst

@@ -78,8 +78,10 @@ configuration of fault-injection capabilities.
 
 - /sys/kernel/debug/fail*/times:
 
-	specifies how many times failures may happen at most.
-	A value of -1 means "no limit".
+	specifies how many times failures may happen at most. A value of -1
+	means "no limit". Note, though, that this file only accepts unsigned
+	values. So, if you want to specify -1, you better use 'printf' instead
+	of 'echo', e.g.: $ printf %#x -1 > times
 
 - /sys/kernel/debug/fail*/space:
 
@@ -167,11 +169,13 @@ configuration of fault-injection capabilities.
 	- ERRNO: retval must be -1 to -MAX_ERRNO (-4096).
 	- ERR_NULL: retval must be 0 or -1 to -MAX_ERRNO (-4096).
 
-- /sys/kernel/debug/fail_function/<functiuon-name>/retval:
+- /sys/kernel/debug/fail_function/<function-name>/retval:
 
-	specifies the "error" return value to inject to the given
-	function for given function. This will be created when
-	user specifies new injection entry.
+	specifies the "error" return value to inject to the given function.
+	This will be created when the user specifies a new injection entry.
+	Note that this file only accepts unsigned values. So, if you want to
+	use a negative errno, you better use 'printf' instead of 'echo', e.g.:
+	$ printf %#x -12 > retval
 
 Boot option
 ^^^^^^^^^^^
@@ -255,7 +259,7 @@ Application Examples
     echo Y > /sys/kernel/debug/$FAILTYPE/task-filter
     echo 10 > /sys/kernel/debug/$FAILTYPE/probability
     echo 100 > /sys/kernel/debug/$FAILTYPE/interval
-    echo -1 > /sys/kernel/debug/$FAILTYPE/times
+    printf %#x -1 > /sys/kernel/debug/$FAILTYPE/times
     echo 0 > /sys/kernel/debug/$FAILTYPE/space
     echo 2 > /sys/kernel/debug/$FAILTYPE/verbose
     echo 1 > /sys/kernel/debug/$FAILTYPE/ignore-gfp-wait
@@ -309,7 +313,7 @@ Application Examples
     echo N > /sys/kernel/debug/$FAILTYPE/task-filter
     echo 10 > /sys/kernel/debug/$FAILTYPE/probability
     echo 100 > /sys/kernel/debug/$FAILTYPE/interval
-    echo -1 > /sys/kernel/debug/$FAILTYPE/times
+    printf %#x -1 > /sys/kernel/debug/$FAILTYPE/times
     echo 0 > /sys/kernel/debug/$FAILTYPE/space
     echo 2 > /sys/kernel/debug/$FAILTYPE/verbose
     echo 1 > /sys/kernel/debug/$FAILTYPE/ignore-gfp-wait
@@ -336,11 +340,11 @@ Application Examples
     FAILTYPE=fail_function
     FAILFUNC=open_ctree
     echo $FAILFUNC > /sys/kernel/debug/$FAILTYPE/inject
-    echo -12 > /sys/kernel/debug/$FAILTYPE/$FAILFUNC/retval
+    printf %#x -12 > /sys/kernel/debug/$FAILTYPE/$FAILFUNC/retval
     echo N > /sys/kernel/debug/$FAILTYPE/task-filter
     echo 100 > /sys/kernel/debug/$FAILTYPE/probability
     echo 0 > /sys/kernel/debug/$FAILTYPE/interval
-    echo -1 > /sys/kernel/debug/$FAILTYPE/times
+    printf %#x -1 > /sys/kernel/debug/$FAILTYPE/times
     echo 0 > /sys/kernel/debug/$FAILTYPE/space
     echo 1 > /sys/kernel/debug/$FAILTYPE/verbose