physio: Don't store user addresses in bio_data

[brooks]

Summary

Only assign the address from the iovec to bio_data if it is a kernel
address. This is the single place where bio_data stored (however briefly)
a userspace pointer.

[brooks]

Cirrus-CI run https://cirrus-ci.com/task/5195860164214784

[brooks]

I needed this change in CheriBSD because we can't store user pointers in bio_data due to a type mismatch and we need to pass the pointer (rather than an address) to vm_fault_quick_hold_pages.