[PATCH v2] fat: Use pointer to simple type in put_user()

[Helge Deller <deller@gmx.de>]

The put_user(val,ptr) macro wants a pointer to a simple type, but in
fat_ioctl_filldir() the d_name field references an "array of chars".
Be more accurate and explicitly give the pointer to the first character
of the d_name[] array.

I noticed that issue while trying to optimize the parisc put_user() macro
and used an intermediate variable to store the pointer. In that case I
got this error:

In file included from include/linux/uaccess.h:11,
                 from include/linux/compat.h:17,
                 from fs/fat/dir.c:18:
fs/fat/dir.c: In function ‘fat_ioctl_filldir’:
fs/fat/dir.c:725:33: error: invalid initializer
  725 |                 if (put_user(0, d2->d_name)                     ||         \
      |                                 ^~
include/asm/uaccess.h:152:33: note: in definition of macro ‘__put_user’
  152 |         __typeof__(ptr) __ptr = ptr;                            \
      |                                 ^~~
fs/fat/dir.c:759:1: note: in expansion of macro ‘FAT_IOCTL_FILLDIR_FUNC’
  759 | FAT_IOCTL_FILLDIR_FUNC(fat_ioctl_filldir, __fat_dirent)


Andreas Schwab <schwab@linux-m68k.org> suggested to use
   __typeof__(&*(ptr)) __ptr = ptr;
instead. This works, but nevertheless it's probably reasonable to
fix the original caller too.

Signed-off-by: Helge Deller <deller@gmx.de>

__
v2: Adjusted the description with input from David Laight
    <David.Laight@ACULAB.COM> and Andreas Schwab <schwab@linux-m68k.org>.

diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index c4a274285858..249825017da7 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -722,7 +722,7 @@ static int func(struct dir_context *ctx, const char *name, int name_len,   \
 		if (name_len >= sizeof(d1->d_name))			   \
 			name_len = sizeof(d1->d_name) - 1;		   \
 									   \
-		if (put_user(0, d2->d_name)			||	   \
+		if (put_user(0, &d2->d_name[0])			||	   \
 		    put_user(0, &d2->d_reclen)			||	   \
 		    copy_to_user(d1->d_name, name, name_len)	||	   \
 		    put_user(0, d1->d_name + name_len)		||	   \