x86/mm: Get rid of conditional IF flag handling in page fault path

We had this nonsensical code that would happily handle kernel page
faults with interrupts disabled, which makes no sense at all.

It turns out that this is legacy code that _used_ to make sense, back
when we enabled IRQs as early as possible, and we used to have this code
sequence essentially immediately after reading the faulting address from
the %cr2 register.

Back then, we could have kernel page faults to populate the vmalloc area
with interrupts disabled, and they would need to stay disabled for that
case.

However, the code in question has been moved down in the page fault
handling, and is now in the "handle faults in user addresses" section,
and apparently nobody ever noticed that it no longer makes sense to
handle these page faults with interrupts conditionally disabled.

So replace the conditional IRQ enable:

        if (regs->flags & X86_EFLAGS_IF)
                local_irq_enable();

with an unconditional one, and add a temporary WARN_ON_ONCE() if some
codepath actually does do page faults with interrupts disabled (without
also doing a pagefault_disable(), of course).

NOTE! We used to allow user space to disable interrupts with iopl(3).
That is no longer true since commits:

 a24ca997 ("x86/iopl: Remove legacy IOPL option")
 b968e84b ("x86/iopl: Fake iopl(3) CLI/STI usage")

so the WARN_ON_ONCE() is valid for both the kernel and user situation.

For some of the history relevant to this code, see particularly commit
8c914cb7

 ("x86_64: actively synchronize vmalloc area when
registering certain callbacks"), which moved this below the vmalloc fault
handling.

Now that the user_mode() check is irrelevant, we can also move the
FAULT_FLAG_USER flag setting down to where the other flag settings are
done.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20240125173457.1281880-1-torvalds@linux-foundation.org