btrfs: remove struct btrfs_bio::is_metadata flag

This flag is unused now, so remove it.  Re-expand the mirror_num field
to 8 bits, and move it to the I/O completion internal section of the
structure.

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>