BACKPORT: 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: 077fbac405bf ("net: xfrm: support setting an output mark.")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
(cherry picked from commit 8598112d04af21cf6c895670e72dcb8a9f58e74f)

Change-Id: I25e9ac6cf79dc8d0ee599bbd23e9d5b5f34a4284