nfscl: Set n_localmodtime in Deallocate

Commit 2be417843a04 added n_localmodtime, which is used by Lookup
and ReaddirPlus to check to see if the file attributes in an RPC
reply might be stale. This patch sets n_localmodtime in Deallocate.
Done as a separate commit, since Deallocate is not in stable/13.

PR: 259071
Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D32635