xsm/flask: Fix sidtab locking bug

xsm/flask:  Fix sidtab locking bug

We do not need to use the _irqsave/irqrestore forms of spin locking
within the sidtab in Xen's XSM Flask module, and doing so triggers a
BUG_ON() within check_lock() when we subsequently call xmalloc().
This was preventing Xen from booting with XSM/Flask enabled if built
with debug=y. It appears that this broke upon the changes to xmalloc
in changeset 18379:14a9a1629590.

Signed-off-by: Stephen D. Smalley <sds@tycho.nsa.gov>
Signed-off-by: George S. Coker, II <gscoker@alpha.ncsc.mil>