svcrpc: set cr_gss_mech from gss-proxy as well as legacy upcall

The change made to rsc_parse() in
0dc1531aca7fd1440918bd55844a054e9c29acad "svcrpc: store gss mech in
svc_cred" should also have been propagated to the gss-proxy codepath.
This fixes a crash in the gss-proxy case.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>