nfsd: Make init_open_stateid() a bit more whole

Move the state selection logic inside from the caller,
always making it return correct stp to use.

Signed-off-by: J . Bruce Fields <bfields@fieldses.org>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>