Change the vm_ooffset_t type to unsigned.

The type represents byte offset in the vm_object_t data space, which
does not span negative offsets in FreeBSD VM. The change matches byte
offset signess with the unsignedness of the vm_pindex_t which
represents the type of the page indexes in the objects.

This allows to remove the UOFF_TO_IDX() macro which was used when we
have to forcibly interpret the type as unsigned anyway. Also it fixes
a lot of implicit bugs in the device drivers d_mmap methods.

Reviewed by: alc, markj (previous version)
Tested by: pho
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation