x86, irq: Rename gsi_end gsi_top, and fix off by one errors

When I introduced the global variable gsi_end I thought gsi_end on
io_apics was one past the end of the gsi range for the io_apic.  After
it was pointed out the the range on io_apics was inclusive I changed
my global variable to match.  That was a big mistake.  Inclusive
semantics without a range start cannot describe the case when no gsi's
are allocated.  Describing the case where no gsi's are allocated is
important in sfi.c and mpparse.c so that we can assign gsi numbers
instead of blindly copying the gsi assignments the BIOS has done as we
do in the acpi case.

To keep from getting the global variable confused with the gsi range
end rename it gsi_top.

To allow describing the case where no gsi's are allocated have gsi_top
be one place the highest gsi number seen in the system.

This fixes an off by one bug in sfi.c:
Reported-by: jacob pan <jacob.jun.pan@linux.intel.com>

This fixes the same off by one bug in mpparse.c:

This fixes an off unreachable by one bug in acpi/boot.c:irq_to_gsi
Reported-by: Yinghai <yinghai.lu@oracle.com>

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
LKML-Reference: <m17hm9jre7.fsf_-_@fess.ebiederm.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>