btrfs: remove spurious BUG_ON in btrfs_get_extent

That BUG_ON cannot ever trigger because as the comment there states -
'err' is always set. Simply remove it as it brings no value.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -6748,7 +6748,6 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,
 		free_extent_map(em);
 		return ERR_PTR(err);
 	}
-	BUG_ON(!em); /* Error is always set */
 	return em;
 }