gmultipath: make physpath distinct from the underlying providers'

zfsd uses a device's physical path attribute to automatically replace a
missing ZFS disk when a blank disk is inserted into the same physical
slot. Currently gmultipath passes through its underlying providers'
physical path attribute. That may cause zfsd to replace a missing
gmultipath provider with a newly arrived, single-path disk. That would
be bad.

This commit fixes that problem by simply appending "/mp" to the
underlying providers' physical path, in a manner similar to what geli
already does.

Sponsored by: Axcient
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D29941