tests: benchmark: app_kernel: Return values from kernel APIs are read.

Static code analysis reported some kernel APIs were used without
reading the return value. Since the benchmark doesn't need error
conditions, a simple read of the values followed by a ARG_UNUSED
is used to handle static code analysis errors.

JIRA: ZEP-2134

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>