btrfs: factor out do_allocation() for extent allocation

Factor out do_allocation() from find_free_extent(). This function do an
actual extent allocation in a given block group. The ffe_ctl->policy is
used to determine the actual allocator function to use.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>