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>