linux-user: complete omission of removing uses of strdup

The 900cfbca just removed two unchecked uses of strdup
in fill_psinfo and missed the rest in core_dump_filename.
This patch fixes it.

Signed-off-by: Wei Jiangang <weijg.fnst@cn.fujitsu.com>
Message-Id: <1459997185-15669-2-git-send-email-weijg.fnst@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>