loader: r316585 did miss userboot update

The work to implement zfs reader to inspect all pool label copies did
miss the userboot, this update does correct this issue.

Since userboot is already using common/disk.c API (disk_open() etc),
the fix is quite simple - we only need to make sure the userdisk_iocl()
will call disk_ioctl(). In addition, the diskioctl callback does return
int, not pointer.

Note, the review request is actually addressing the sparc and userboot,
but as testing the fix for sparc will take some more time, I am posting the
userboot fix now.

This patch is part of the implementation presented in review:
https://reviews.freebsd.org/D10302

Once we have the sparc part tested, we will have the complete fix
for the issue.

Reviewed by: imp