panic: add option to dump all CPUs backtraces in panic_print

Currently the "panic_print" parameter/sysctl allows some interesting debug
information to be printed during a panic event.  This is useful for
example in cases the user cannot kdump due to resource limits, or if the
user collects panic logs in a serial output (or pstore) and prefers a fast
reboot instead of a kdump.

Happens that currently there's no way to see all CPUs backtraces in a
panic using "panic_print" on architectures that support that.  We do have
"oops_all_cpu_backtrace" sysctl, but although partially overlapping in the
functionality, they are orthogonal in nature: "panic_print" is a panic
tuning (and we have panics without oopses, like direct calls to panic() or
maybe other paths that don't go through oops_enter() function), and the
original purpose of "oops_all_cpu_backtrace" is to provide more
information on oopses for cases in which the users desire to continue
running the kernel even after an oops, i.e., used in non-panic scenarios.

So, we hereby introduce an additional bit for "panic_print" to allow
dumping the CPUs backtraces during a panic event.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Guilherme G. Piccoli <[email protected]>
Reviewed-by: Feng Tang <[email protected]>
Cc: Iurii Zaikin <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Samuel Iglesias Gonsalvez <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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