btrfs: make create_io_em take btrfs_inode

It really wants a btrfs_inode and will allow submit_compressed_extents
to be completely converted to btrfs_inode in follow up patches.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>