spec: bind mount /sys only when userNS are enabled

Fix the test for checking when /sys must be bind mounted from the
host.  It should be done only when userNS are enabled (the
!UsernsMode.IsHost() check is not enough for that).

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #1349
Approved by: rhatdan
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