x86/IOMMU: Use altcall, and __initconst_cf_clobber

x86/IOMMU: Use altcall, and __initconst_cf_clobber

Most IOMMU hooks are already altcall for performance reasons.  Convert the
rest of them so we can harden all the hooks in Control Flow Integrity
configurations.  This necessitates the use of iommu_{v,}call() in debug builds
too.  Switch to using an ASSERT() as all forms should resolve to &iommu_ops.

Move the root iommu_ops from __read_mostly to __ro_after_init now that the
latter exists.

Since c/s 3330013e6739 ("VT-d / x86: re-arrange cache syncing"), vtd_ops is
not modified and doesn't need a forward declaration, so we can use
__initconst_cf_clobber for both VT-d and AMD.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>