fs: Add helper to clean bdev aliases under a bh and use it

Add a helper function that clears buffer heads from a block device
aliasing passed bh. Use this helper function from filesystems instead of
the original unmap_underlying_metadata() to save some boiler plate code
and also have a better name for the functionalily since it is not
unmapping anything for a *long* time.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fs/mpage.c b/fs/mpage.c
index 98fc11a..28af984 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -555,8 +555,7 @@ static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 		if (mpd->get_block(inode, block_in_file, &map_bh, 1))
 			goto confused;
 		if (buffer_new(&map_bh))
-			unmap_underlying_metadata(map_bh.b_bdev,
-						map_bh.b_blocknr);
+			clean_bdev_bh_alias(&map_bh);
 		if (buffer_boundary(&map_bh)) {
 			boundary_block = map_bh.b_blocknr;
 			boundary_bdev = map_bh.b_bdev;