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>

include/linux/sunrpc/gss_api.h

@@ -151,6 +151,8 @@ struct gss_api_mech *gss_mech_get_by_pseudoflavor(u32);
 /* Fill in an array with a list of supported pseudoflavors */
 int gss_mech_list_pseudoflavors(rpc_authflavor_t *, int);
 
+struct gss_api_mech * gss_mech_get(struct gss_api_mech *);
+
 /* For every successful gss_mech_get or gss_mech_get_by_* call there must be a
  * corresponding call to gss_mech_put. */
 void gss_mech_put(struct gss_api_mech *);