nfs_clvfsops.c: Allow long directory mount paths

Without this patch, the NFS client code for mounts has
an arbitrary 100 character limit for the directory path
being mounted on the server.

Someone reported this quite a while ago, but I cannot
find the email or bugzilla PR.

This patch fixes the problem by increasing the directory
path length limit to MNAMELEN, allocated via malloc() to
avoid using too much stack.

MFC after: 2 weeks