ARM: entry: fix unwinder problems caused by IRQ stacks

ARM: entry: fix unwinder problems caused by IRQ stacks

The IRQ stacks series made some changes to the unwinder, to permit
unwinding across different stacks. This is needed because otherwise, the
call stack would terminate at the point where the stack switch between
the task stack and the IRQ stack occurs, which would defeat any
diagnostics that rely on timer interrupts, such as RCU stall detection.

Unfortunately, getting the unwind annotations correct turns out to be
difficult, given that this now involves a frame pointer which needs to
point into the right location in the task stack when unwinding from the
IRQ stack. Getting this wrong for an exception handling routine results
in the stack pointer to be unwound from the wrong location, causing any
subsequent unwind attempts to cause all kinds of issues, as reported by
Naresh here [0].

So let's simplify this, by deferring the stack switch to
call_with_stack(), which already has the correct unwind annotations, and
removing all the complicated handling of the stack frame from the IRQ
exception entrypoint itself.

[0] https://lore.kernel.org/all/CA+G9fYtpy8VgK+ag6OsA9TDrwi5YGU4hu7GM8xwpO7v6LrCD4Q@mail.gmail.com/

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>