nfs_nfsdserv.c: Add sanity check for layout commit cnt

[rmacklem]

Summary

If a client were to send a LayoutCommit (seldom
used and only for a pNFS server) with a bogus
cnt, there could be problems with a malloc() call
that uses it.

This patch adds a sanity check for the cnt. Note
that RFC8881 does not specify any upper bound
on the cnt.

Test Plan

Not tested. To do so would require a pNFS server
configuration and a broken client.