xfrm: Add error handling when nla_put_u32() returns an error

Error handling is missing when call to nla_put_u32() fails.
Handle the error when the call to nla_put_u32() returns an error.

The error was reported by Coverity Scan.
Report:
CID 1601525: (#1 of 1): Unused value (UNUSED_VALUE)
returned_value: Assigning value from nla_put_u32(skb, XFRMA_SA_PCPU, x->pcpu_num)
to err here, but that stored value is overwritten before it can be used

Fixes: 1ddf9916 ("xfrm: Add support for per cpu xfrm state handling.")
Signed-off-by: Everest K.C. <everestkc@everestkc.com.np>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>