xfrm: Try to honor policy index if it's supplied by user

xfrm code always searches for unused policy index for
newly created policy regardless whether or not user
space policy index hint supplied.

This patch enables such feature so that using
"ip xfrm ... index=xxx" can be used by user to set
specific policy index.

Currently this beahvior is broken, so this patch make
it happen as expected.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>