panic, kexec: make __crash_kexec() NMI safe

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2166717 Upstream-status: https://github.com/torvalds/linux.git

Using a mutex to synchronize access to the crash kernel image is very much not NMI safe, these patches address that.

Tested: Triggered NMIs via ipmitool using the following script:

echo 1 > /proc/sys/kernel/panic_on_unrecovered_nmi
echo 1 > /proc/sys/kernel/unknown_nmi_panic
echo 1 > /proc/sys/kernel/panic

ipmitool power diag

Signed-off-by: Valentin Schneider vschneid@redhat.com