xfrm: fix the condition on x->pcpu_num in xfrm_sa_len

pcpu_num = 0 is a valid value. The marker for "unset pcpu_num" which
makes copy_to_user_state_extra not add the XFRMA_SA_PCPU attribute is
UINT_MAX.

Fixes: 1ddf9916ac09 ("xfrm: Add support for per cpu xfrm state handling.")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>