fscrypt: mention init_on_free instead of page poisoning

Page poisoning is an older debug option.  The modern way to initialize
memory on free for security reasons is to set init_on_free=1.

Link: https://lore.kernel.org/r/20250304210156.14912-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

Author: ebiggers

Documentation/filesystems/fscrypt.rst

@@ -137,9 +137,8 @@ However, these ioctls have some limitations:
 - In general, decrypted contents and filenames in the kernel VFS
   caches are freed but not wiped.  Therefore, portions thereof may be
   recoverable from freed memory, even after the corresponding key(s)
-  were wiped.  To partially solve this, you can set
-  CONFIG_PAGE_POISONING=y in your kernel config and add page_poison=1
-  to your kernel command line.  However, this has a performance cost.
+  were wiped.  To partially solve this, you can add init_on_free=1 to
+  your kernel command line.  However, this has a performance cost.
 
 - Secret keys might still exist in CPU registers, in crypto
   accelerator hardware (if used by the crypto API to implement any of