btrfs: reduce space_info critical section in btrfs_chunk_alloc()

btrfs: reduce space_info critical section in btrfs_chunk_alloc()

There's no need to update local variables while holding the space_info's
spinlock, since the update isn't using anything from the space_info. So
move these updates outside the critical section to shorten it.

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