arm64: fix wrong check of on_sdei_stack in nmi context

arm64: fix wrong check of on_sdei_stack in nmi context

When doing unwind_frame() in the context of pseudo nmi (need enable
CONFIG_ARM64_PSEUDO_NMI), reaching the bottom of the stack (fp == 0,
pc != 0), function on_sdei_stack() will return true while the sdei acpi
table is not inited in fact. This will cause a "NULL pointer dereference"
oops when going on.

Reviewed-by: Julien Thierry <[email protected]>
Signed-off-by: Wei Li <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>