sunrpc: Extract target name into svc_cred

NFSv4.0 callback needs to know the GSS target name the client used
when it established its lease. That information is available from
the GSS context created by gssproxy. Make it available in each
svc_cred.

Note this will also give us access to the real target service
principal name (which is typically "nfs", but spec does not require
that).

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>