NFSv2: Fix a typo in encode_sattr()

commit ad97a995 upstream.

Encode the mtime correctly.

Fixes: 95582b00 ("vfs: change inode times to use struct timespec64")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>