btrfs: Remove redundant FLAG_VACANCY

Commit 9036c10208e1 ("Btrfs: update hole handling v2") added the
FLAG_VACANCY to denote holes, however there was already a consistent way
of flagging extents which represent hole - ->block_start =
EXTENT_MAP_HOLE. And also the only place where this flag is checked is
in the fiemap code, but the block_start value is also checked and every
other place in the filesystem detects holes by using block_start
value's. So remove the extra flag. This survived a full xfstest run.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 932d805..ef72efe 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4311,10 +4311,8 @@ static struct extent_map *get_extent_skip_holes(struct inode *inode,
 			return em;
 
 		/* if this isn't a hole return it */
-		if (!test_bit(EXTENT_FLAG_VACANCY, &em->flags) &&
-		    em->block_start != EXTENT_MAP_HOLE) {
+		if (em->block_start != EXTENT_MAP_HOLE)
 			return em;
-		}
 
 		/* this is a hole, advance to the next extent */
 		offset = extent_map_end(em);