gpart: append partition name to the underlying provider's physical path

If the underlying provider's physical path is null, then the gpart device's
physical path will be, too. Otherwise, it will append the partition name,
such as "/p1" or "/s1/a". This will make gpart work better with zfsd(8).

PR: 224965
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D14010