f2fs: add missing argument to f2fs_setxattr stub

The f2fs_setxattr() prototype for CONFIG_F2FS_FS_XATTR=n has
been wrong for a long time, since 8ae8f162 ("f2fs: support
xattr security labels"), but there have never been any callers,
so it did not matter.

Now, the function gets called from f2fs_ioc_keyctl(), which
causes a build failure:

fs/f2fs/file.c: In function 'f2fs_ioc_keyctl':
include/linux/stddef.h:7:14: error: passing argument 6 of 'f2fs_setxattr' makes integer from pointer without a cast [-Werror=int-conversion]
 #define NULL ((void *)0)
              ^
fs/f2fs/file.c:1599:27: note: in expansion of macro 'NULL'
     value, F2FS_KEY_SIZE, NULL, type);
                           ^
In file included from ../fs/f2fs/file.c:29:0:
fs/f2fs/xattr.h:129:19: note: expected 'int' but argument is of type 'void *'
 static inline int f2fs_setxattr(struct inode *inode, int index,
                   ^
fs/f2fs/file.c:1597:9: error: too many arguments to function 'f2fs_setxattr'
  return f2fs_setxattr(inode, F2FS_XATTR_INDEX_KEY,
         ^
In file included from ../fs/f2fs/file.c:29:0:
fs/f2fs/xattr.h:129:19: note: declared here
 static inline int f2fs_setxattr(struct inode *inode, int index,

Thsi changes the prototype of the empty stub function to match
that of the actual implementation. This will not make the key
management work when F2FS_FS_XATTR is disabled, but it gets it
to build at least.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>