krpc: Fix NFS-over-TLS for KTLS1.3

When NFS-over-TLS uses KTLS1.3, the client can receive
post-handshake handshake records. These records can be
safely thown away, but are not handled correctly via the
rpctls_ct_handlerecord() upcall to the daemon.

Commit 373511338d95 changed soreceive_generic() so that it
will only return ENXIO for Alert records when MSG_TLSAPPDATA
is specified. As such, the post-handshake handshake
records will be returned to the krpc.

This patch modifies the krpc so that it will throw
these records away, which seems sufficient to make
NFS-over-TLS work with KTLS1.3. This change has
no effect on the use of KTLS1.2, since it does not
generate post-handshake handshake records.

MFC after: 2 weeks