arm64: efi: Restore register x18 if it was corrupted

mainline inclusion
from mainline-v5.8-rc1
commit e73f02c6
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I67RIP
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e73f02c6eb15729164b9dd5e19214b54446823ab



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If we detect a corrupted x18, restore the register before jumping back
to potentially SCS instrumented code. This is safe, because the wrapper
is called with preemption disabled and a separate shadow stack is used
for interrupt handling.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Ding Hui <dinghui@sangfor.com.cn>