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>