btrfs: reduce block group critical section in pin_down_extent()

btrfs: reduce block group critical section in pin_down_extent()

There's no need to update the bytes_reserved and bytes_may_use fields of
the space_info while holding the block group's spinlock. We are only
making the critical section longer than necessary. So move the space_info
updates outside of the block group's critical section.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>