xfrm: Fix return value check of copy_sec_ctx.

A recent commit added an output_mark. When copying
this output_mark, the return value of copy_sec_ctx
is overwitten without a check. Fix this by copying
the output_mark before the security context.

Fixes: 077fbac4 ("net: xfrm: support setting an output mark.")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>