block: Add fallthrough markers to switch statements

This patch suppresses gcc 7 warnings about falling through in switch
statements when building with W=1. From the gcc documentation: The
-Wimplicit-fallthrough=3 warning is enabled by -Wextra. See also
https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/Warning-Options.html.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/ioprio.c b/block/ioprio.c
index 4b120c9..6f5d0b6 100644
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -75,7 +75,8 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 		case IOPRIO_CLASS_RT:
 			if (!capable(CAP_SYS_ADMIN))
 				return -EPERM;
-			/* fall through, rt has prio field too */
+			/* fall through */
+			/* rt has prio field too */
 		case IOPRIO_CLASS_BE:
 			if (data >= IOPRIO_BE_NR || data < 0)
 				return -EINVAL;