btrfs: move reclaiming of a single block group into its own function

btrfs: move reclaiming of a single block group into its own function

The main work of reclaiming a single block-group in
btrfs_reclaim_bgs_work() is done inside the loop iterating over all the
block_groups in the fs_info->reclaim_bgs list.

Factor out reclaim of a single block group from the loop to improve
readability.

No functional change intended.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>