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>

fs/ocfs2/aops.c

@@ -630,7 +630,7 @@ int ocfs2_map_page_blocks(struct page *page, u64 *p_blkno,
 
 		if (!buffer_mapped(bh)) {
 			map_bh(bh, inode->i_sb, *p_blkno);
-			unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
+			clean_bdev_bh_alias(bh);
 		}
 
 		if (PageUptodate(page)) {