string: add mem_is_zero() helper to check if memory area is all zeros

Almost two thirds of the memchr_inv() usages check if the memory area is
all zeros, with no interest in where in the buffer the first non-zero
byte is located. Checking for !memchr_inv(s, 0, n) is also not very
intuitive or discoverable. Add an explicit mem_is_zero() helper for this
use case.

Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240814100035.3100852-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

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