nfsd: Fix cred ref leak in nfsd_nl_listener_set_doit().

In the Linux kernel, the following vulnerability has been resolved:

nfsd: Fix cred ref leak in nfsd_nl_listener_set_doit().

nfsd_nl_listener_set_doit() uses get_current_cred() without

put_cred().

As we can see from other callers, svc_xprt_create_from_sa()

does not require the extra refcount.

nfsd_nl_listener_set_doit() is always in the process context,

sendmsg(), and current->cred does not go away.

Let's use current_cred() in nfsd_nl_listener_set_doit().