include/linux/fs.h: fix unsigned enum warning with gcc-4.2

With arm-linux-gcc-4.2, almost every file we build in the kernel ends up
with this warning:

  include/linux/fs.h:2648: warning: comparison of unsigned expression < 0 is always false

Later versions don't have this problem, but it's easy enough to work
around.

Link: http://lkml.kernel.org/r/20161216105634.235457-12-arnd@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/fs.h

@@ -2678,7 +2678,7 @@ static const char * const kernel_read_file_str[] = {
 
 static inline const char *kernel_read_file_id_str(enum kernel_read_file_id id)
 {
-	if (id < 0 || id >= READING_MAX_ID)
+	if ((unsigned)id >= READING_MAX_ID)
 		return kernel_read_file_str[READING_UNKNOWN];
 
 	return kernel_read_file_str[id];