fs: fix over-zealous use of "const"

When I was fixing up const recommendations from checkpatch.pl, I went
overboard. This fixes the warning (during a W=1 build):

include/linux/fs.h:2627:74: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
static inline const char * const kernel_read_file_id_str(enum kernel_read_file_id id)
Reported-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NJames Morris <james.l.morris@oracle.com>

include/linux/fs.h

@@ -2600,7 +2600,7 @@ static const char * const kernel_read_file_str[] = {
 	__kernel_read_file_id(__fid_stringify)
 };
 
-static inline const char * const kernel_read_file_id_str(enum kernel_read_file_id id)
+static inline const char *kernel_read_file_id_str(enum kernel_read_file_id id)
 {
 	if (id < 0 || id >= READING_MAX_ID)
 		return kernel_read_file_str[READING_UNKNOWN];