nfs_commonsubs.c: Get rid of variable used as a constant

The variable ncl_mbuf_mhlen is set to MHLEN and only
used in one place. This patch simply deletes it and
replaces it with MHLEN.

This patch should not affect semantics.

Spotted while fixing cases that could do large
NFSM_DISSECT() sizes.

MFC after: 2 weeks