btrfs: consolidate bitmap_clear_bits/__bitmap_clear_bits

The only difference is the former adjusts btrfs_free_space::bytes
member. Consolidate the two function into 1 and add a bool parameter
which controls whether the adjustment is made or not. No functional
changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>