treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index a12f42e..e732580 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1181,7 +1181,7 @@ int nfs_lookup_verify_inode(struct inode *inode, unsigned int flags)
 			/* A NFSv4 OPEN will revalidate later */
 			if (server->caps & NFS_CAP_ATOMIC_OPEN)
 				goto out;
-			/* Fallthrough */
+			fallthrough;
 		case S_IFDIR:
 			if (server->flags & NFS_MOUNT_NOCTO)
 				break;