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/buffer.c b/fs/buffer.c
index d468ed9..50bbc99 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1958,7 +1958,7 @@ iomap_to_bh(struct inode *inode, sector_t block, struct buffer_head *bh,
 		 */
 		set_buffer_new(bh);
 		set_buffer_unwritten(bh);
-		/* FALLTHRU */
+		fallthrough;
 	case IOMAP_MAPPED:
 		if ((iomap->flags & IOMAP_F_NEW) ||
 		    offset >= i_size_read(inode))