btrfs: cache RAID stripe tree decision in btrfs_io_context

Cache the decision if a particular I/O needs to update RAID stripe tree
entries in struct btrfs_io_context.

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