gnttab: make sure grant map operations don't skip their IOMMU part

gnttab: make sure grant map operations don't skip their IOMMU part

Two almost simultaneous mapping requests need to make sure that at the
completion of the earlier one IOMMU mappings (established explicitly
here in the PV case) have been put in place. Forever since the splitting
of the grant table lock a violation of this has been possible (using
simplified pin counts, as it doesn't matter whether we talk about read
or write mappings here):

initial state: act->pin = 0

vCPU A: progress the operation past the dropping of the locks after the
        act->pin updates (act->pin = 1, old_pin = 0, act_pin = 1)

vCPU B: progress the operation past the dropping of the locks after the
        act->pin updates (act->pin = 2, old_pin = 1, act_pin = 2)

vCPU B: (re-)acquire both gt locks, mapkind() returns 0, but both
        iommu_legacy_map() invocations get skipped due to non-zero
        old_pin

vCPU B: return to caller without IOMMU mapping

vCPU A: (re-)acquire both gt locks, mapkind() returns 0,
        iommu_legacy_map() gets invoked

With the locks dropped intermediately, whether to invoke
iommu_legacy_map() must depend on only the return value of mapkind()
and of course the kind of mapping request being processed, just like
is already the case in unmap_common().

Also fix the style of the adjacent comment, and correct a nearby one
still referring to a prior name of what is now mapkind().

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