[XFRM]: remove xerr_idxp from __xfrm_policy_check()

It seems that during the MIPv6 respin, some code which was originally
conditionally compiled around CONFIG_XFRM_ADVANCED was accidently left
in after the config option was removed.

This patch removes an extraneous pointer (xerr_idxp) which is no
longer needed.

Signed-off-by: James Morris <jmorris@namei.org>
Acked-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>