svcrpc: store gss mech in svc_cred

Store a pointer to the gss mechanism used in the rq_cred and cl_cred.
This will make it easier to enforce SP4_MACH_CRED, which needs to
compare the mechanism used on the exchange_id with that used on
protected operations.

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