SUNRPC: Add a missing spin_unlock to gss_mech_list_pseudoflavors

The patch "SUNRPC: Add rpcauth_list_flavors()" introduces a new error
path in gss_mech_list_pseudoflavors, but fails to release the spin lock.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth_gss/gss_mech_switch.c

@@ -257,8 +257,10 @@ int gss_mech_list_pseudoflavors(rpc_authflavor_t *array_ptr, int size)
 	spin_lock(&registered_mechs_lock);
 	list_for_each_entry(pos, &registered_mechs, gm_list) {
 		for (j = 0; j < pos->gm_pf_num; j++) {
-			if (i >= size)
+			if (i >= size) {
+				spin_unlock(&registered_mechs_lock);
 				return -ENOMEM;
+			}
 			array_ptr[i++] = pos->gm_pfs[j].pseudoflavor;
 		}
 	}