arm64: move smp_send_stop() cpu mask off stack

arm64: move smp_send_stop() cpu mask off stack

For really large values of CONFIG_NR_CPUS, a CPU mask value should
not be put on the stack:

arch/arm64/kernel/smp.c:1188:1: error: the frame size of 8544 bytes is larger than 1536 bytes [-Werror=frame-larger-than=]

This could be achieved using alloc_cpumask_var(), which makes it
depend on CONFIG_CPUMASK_OFFSTACK, but as this function is already
serialized and can only run on one CPU, making the variable 'static'
is easier.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20250620111045.3364827-1-arnd@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>