MFC 212293:

MFC 212293:
Store the full timestamp when caching timestamps of files and
directories for purposes of validating name cache entries.  This
closes races where two updates to a file or directory within the same
second could result in stale entries in the name cache.

To preserve the ABI of 'struct nfsnode', the existing timestamp fields
are left with 'n_unusedX' placeholders along with the unused 'n_expiry'
field.  The larger n_ctime and n_dmtime fields are added to the end of
the structure.

git-svn-id: svn://svn.freebsd.org/base/stable/8@212929 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f