btrfs: add and use helper to determine if using bitmaps in free space tree

btrfs: add and use helper to determine if using bitmaps in free space tree

When adding and removing free space to the free space tree, we need to
lookup the respective block group's free info item in the free space tree,
check its flags for the BTRFS_FREE_SPACE_USING_BITMAPS bit and then
release the path.

Move these steps into a helper function and use it in both sites.
This will also help avoiding duplicate code in a subsequent change.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>