fcntl: Fix unreachable code in do_fcntl()

Fix the following warning:

fs/fcntl.c:373:3: warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
                   fallthrough;
                   ^
   include/linux/compiler_attributes.h:210:41: note: expanded from macro 'fallthrough'
   # define fallthrough                    __attribute__((__fallthrough__))

by placing the fallthrough; statement inside ifdeffery.
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>

fs/fcntl.c

@@ -369,8 +369,8 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	/* 32-bit arches must use fcntl64() */
 	case F_OFD_SETLK:
 	case F_OFD_SETLKW:
-#endif
 		fallthrough;
+#endif
 	case F_SETLK:
 	case F_SETLKW:
 		if (copy_from_user(&flock, argp, sizeof(flock)))