NFS: Further fixes to attribute delegation a/mtime changes

When asked to set both an atime and an mtime to the current system time,
ensure that the setting is atomic by calling inode_update_timestamps()
only once with the appropriate flags.

Fixes: e12912d9 ("NFSv4: Add support for delegated atime and mtime attributes")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>