x86/flushtlb: remove flush_area check on system state

x86/flushtlb: remove flush_area check on system state

Booting with Shadow Stacks leads to the following assert on a debug
hypervisor:

Assertion 'local_irq_is_enabled()' failed at arch/x86/smp.c:265
----[ Xen-4.17.0-10.24-d  x86_64  debug=y  Not tainted ]----
CPU:    0
RIP:    e008:[<ffff82d040345300>] flush_area_mask+0x40/0x13e
[...]
Xen call trace:
   [<ffff82d040345300>] R flush_area_mask+0x40/0x13e
   [<ffff82d040338a40>] F modify_xen_mappings+0xc5/0x958
   [<ffff82d0404474f9>] F arch/x86/alternative.c#_alternative_instructions+0xb7/0xb9
   [<ffff82d0404476cc>] F alternative_branches+0xf/0x12
   [<ffff82d04044e37d>] F __start_xen+0x1ef4/0x2776
   [<ffff82d040203344>] F __high_start+0x94/0xa0

This is due to SYS_STATE_smp_boot being set before calling
alternative_branches(), and the flush in modify_xen_mappings() then
using flush_area_all() with interrupts disabled.  Note that
alternative_branches() is called before APs are started, so the flush
must be a local one (and indeed the cpumask passed to
flush_area_mask() just contains one CPU).

Take the opportunity to simplify a bit the logic and make flush_area()
an alias of flush_area_all() in mm.c, taking into account that
cpu_online_map just contains the BSP before APs are started.  This
requires widening the assert in flush_area_mask() to allow being
called with interrupts disabled as long as it's strictly a local only
flush.

The overall result is that a conditional can be removed from
flush_area().

While there also introduce an ASSERT to check that a vCPU state flush
is not issued for the local CPU only.

Fixes: 78e072bc37 ('x86/mm: avoid inadvertently degrading a TLB flush to local only')
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>