nfsd: Disable NFSv2 timestamp workaround for NFSv3+

NFSv2 can set the atime and/or mtime of a file to specific timestamps but not
to the server's current time.  To implement the equivalent of utimes("file",
NULL), it uses a heuristic.

NFSv3 and later do support setting the atime and/or mtime to the server's
current time directly.  The NFSv2 heuristic is still enabled, and causes
timestamps to be set wrong sometimes.

Fix this by moving the heuristic into the NFSv2 specific code.  We can leave it
out of the create code path: the owner can always set timestamps arbitrarily,
and the workaround would never trigger.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>