selftests: xfrm: fix test return value override issue in xfrm_policy.sh

When running this xfrm_policy.sh test script, even with some cases
marked as FAIL, the overall test result will still be PASS:

$ sudo ./xfrm_policy.sh
PASS: policy before exception matches
FAIL: expected ping to .254 to fail (exceptions)
PASS: direct policy matches (exceptions)
PASS: policy matches (exceptions)
FAIL: expected ping to .254 to fail (exceptions and block policies)
PASS: direct policy matches (exceptions and block policies)
PASS: policy matches (exceptions and block policies)
FAIL: expected ping to .254 to fail (exceptions and block policies after hresh changes)
PASS: direct policy matches (exceptions and block policies after hresh changes)
PASS: policy matches (exceptions and block policies after hresh changes)
FAIL: expected ping to .254 to fail (exceptions and block policies after hthresh change in ns3)
PASS: direct policy matches (exceptions and block policies after hthresh change in ns3)
PASS: policy matches (exceptions and block policies after hthresh change in ns3)
FAIL: expected ping to .254 to fail (exceptions and block policies after htresh change to normal)
PASS: direct policy matches (exceptions and block policies after htresh change to normal)
PASS: policy matches (exceptions and block policies after htresh change to normal)
PASS: policies with repeated htresh change
$ echo $?
0

This is because the $lret in check_xfrm() is not a local variable.
Therefore when a test failed in check_exceptions(), the non-zero $lret
will later get reset to 0 when the next test calls check_xfrm().

With this fix, the final return value will be 1. Make it easier for
testers to spot this failure.

Fixes: 39aa6928 ("xfrm: policy: fix netlink/pf_key policy lookups")
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>