Fix nfsmount() so that it will return NFSERR_MINORVERMISMATCH.

If nfsrpc_getdirpath() returns NFSERR_MINORVERMISMATCH, it would erroneously
get mapped to EIO. This was not particularily harmful, but would make it
hard for sysadmins to diagnose why an NFSv4 mount is failing.

mount_nfs.c still needs to be fixed so that it does not report
NFSERR_MINORVERMISMATCH as an unknown error 10021.

MFC after: 1 week