x86/boot: Fix early exception handling with CONFIG_PERF_COUNTERS

x86/boot: Fix early exception handling with CONFIG_PERF_COUNTERS

The PERFC_INCR() macro uses current->processor, but current is not valid
during early boot.  This causes the following crash to occur if
e.g. rdmsr_safe() has to recover from a #GP fault.

  (XEN) Early fatal page fault at e008:ffff82d0803b1a39 (cr2=0000000000000004, ec=0000)
  (XEN) ----[ Xen-4.14-unstable  x86_64  debug=y   Not tainted ]----
  (XEN) CPU:    0
  (XEN) RIP:    e008:[<ffff82d0803b1a39>] x86_64/entry.S#handle_exception_saved+0x64/0xb8
  ...
  (XEN) Xen call trace:
  (XEN)    [<ffff82d0803b1a39>] R x86_64/entry.S#handle_exception_saved+0x64/0xb8
  (XEN)    [<ffff82d0806394fe>] F __start_xen+0x2cd/0x2980
  (XEN)    [<ffff82d0802000ec>] F __high_start+0x4c/0x4e

Furthermore, the PERFC_INCR() macro is wildly inefficient.  There has been a
single caller for many releases now, so inline it and delete the macro
completely.

There is no need to reference current at all.  What is actually needed is the
per_cpu_offset which can be obtained directly from the top-of-stack block.
This simplifies the counter handling to 3 instructions and no spilling to the
stack at all.

The same breakage from above is now handled properly:

  (XEN) traps.c:1591: GPF (0000): ffff82d0806394fe [__start_xen+0x2cd/0x2980] -> ffff82d0803b3bfb

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Julien Grall <jgrall@amazon.com>