mm: use fallthrough;

Convert the various /* fallthrough */ comments to the pseudo-keyword
fallthrough;

Done via script:
https://lore.kernel.org/lkml/b56602fcf79f849e733e7b521bb0e17895d390fa.1582230379.git.joe@perches.com/

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: http://lkml.kernel.org/r/f62fea5d10eb0ccfc05d87c242a620c261219b66.camel@perches.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/gup.c b/mm/gup.c
index b185377..a212305 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1102,7 +1102,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 				goto retry;
 			case -EBUSY:
 				ret = 0;
-				/* FALLTHRU */
+				fallthrough;
 			case -EFAULT:
 			case -ENOMEM:
 			case -EHWPOISON: