nfscl: Add setting n_localmodtime to the Write RPC code

Similar to commit 2be417843a, I believe there could be a race between
the NFS client VOP_LOOKUP() and file Writing that could result in stale
file attributes being loaded into the NFS vnode by VOP_LOOKUP().

I have not been able to reproduce a failure due to this race, but
I believe that there are two possibilities:

The Lookup RPC happens while VOP_WRITE() is being executed and loads
stale file attributes after VOP_WRITE() returns when it has already
completed the Write/Commit RPC(s).
--> For this case, setting the local modify timestamp at the end of

VOP_WRITE() should ensure that stale file attributes are not loaded.

The Lookup RPC occurs after VOP_WRITE() has returned, while
asynchronous Write/Commit RPCs are in progress and then is
blocked by the vnode held by VOP_OPEN/VOP_CLOSE/VOP_FSYNC which
will flush writes via ncl_flush() or ncl_vinvalbuf(), clearing the
NMODIFIED flag (which indicates Writes-in-progress). The VOP_LOOKUP()
then acquires the NFS vnode lock and fills in stale file attributes.
--> Setting the local modify timestamp in ncl_flsuh() and ncl_vinvalbuf()

when they clear NMODIFIED should ensure that stale file attributes
are not loaded.

This patch does the above.

PR: 259071
(cherry picked from commit 50dcff0816e5e4aa94f51ce27da5121e49902996)