physio: Don't store user addresses in bio_data

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

Reviewed by: imp, markj
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26783