readdir: Replace one-element arrays with flexible-array members

One-element arrays are deprecated, and we are replacing them with flexible
array members instead. So, replace one-element arrays with flexible-array
members in multiple structures.

Address the following -Wstringop-overflow warnings seen when built
m68k architecture with m5307c3_defconfig configuration:
In function '__put_user_fn',
    inlined from 'fillonedir' at fs/readdir.c:170:2:
include/asm-generic/uaccess.h:49:35: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
   49 |                 *(u8 __force *)to = *(u8 *)from;
      |                 ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
fs/readdir.c: In function 'fillonedir':
fs/readdir.c:134:25: note: at offset 1 into destination object 'd_name' of size 1
  134 |         char            d_name[1];
      |                         ^~~~~~
In function '__put_user_fn',
    inlined from 'filldir' at fs/readdir.c:257:2:
include/asm-generic/uaccess.h:49:35: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
   49 |                 *(u8 __force *)to = *(u8 *)from;
      |                 ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
fs/readdir.c: In function 'filldir':
fs/readdir.c:211:25: note: at offset 1 into destination object 'd_name' of size 1
  211 |         char            d_name[1];
      |                         ^~~~~~

This helps with the ongoing efforts to globally enable
-Wstringop-overflow.

This results in no differences in binary output.

Link: KSPP#79
Link: KSPP#312
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Message-Id: <ZJHiPJkNKwxkKz1c@work>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: GustavoARSilva

fs/readdir.c

@@ -131,7 +131,7 @@ struct old_linux_dirent {
 	unsigned long	d_ino;
 	unsigned long	d_offset;
 	unsigned short	d_namlen;
-	char		d_name[1];
+	char		d_name[];
 };
 
 struct readdir_callback {
@@ -208,7 +208,7 @@ struct linux_dirent {
 	unsigned long	d_ino;
 	unsigned long	d_off;
 	unsigned short	d_reclen;
-	char		d_name[1];
+	char		d_name[];
 };
 
 struct getdents_callback {
@@ -388,7 +388,7 @@ struct compat_old_linux_dirent {
 	compat_ulong_t	d_ino;
 	compat_ulong_t	d_offset;
 	unsigned short	d_namlen;
-	char		d_name[1];
+	char		d_name[];
 };
 
 struct compat_readdir_callback {
@@ -460,7 +460,7 @@ struct compat_linux_dirent {
 	compat_ulong_t	d_ino;
 	compat_ulong_t	d_off;
 	unsigned short	d_reclen;
-	char		d_name[1];
+	char		d_name[];
 };
 
 struct compat_getdents_callback {