btrfs: optimize simple reads in btrfsic_map_block

Pass a smap into __btrfs_map_block so that the usual case of a read that
doesn't require parity raid recovery doesn't need an extra memory
allocation for the btrfs_io_context.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>