arm64/sysreg: Fix broken strncpy() -> strscpy() conversion

arm64/sysreg: Fix broken strncpy() -> strscpy() conversion

Mostafa reports that commit d232606773a0 ("arm64/sysreg: refactor
deprecated strncpy") breaks our early command-line parsing because the
original code is working on space-delimited substrings rather than
NUL-terminated strings.

Rather than simply reverting the broken conversion patch, replace the
strscpy() with a simple memcpy() with an explicit NUL-termination of the
result.

Reported-by: Mostafa Saleh <smostafa@google.com>
Tested-by: Mostafa Saleh <smostafa@google.com>
Fixes: d232606773a0 ("arm64/sysreg: refactor deprecated strncpy")
Signed-off-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230905-strncpy-arch-arm64-v4-1-bc4b14ddfaef@google.com
Link: https://lore.kernel.org/r/20230831162227.2307863-1-smostafa@google.com
Signed-off-by: Will Deacon <will@kernel.org>