kselftest: save-and-restore errno to allow for %m formatting

Previously, using "%m" in a ksft_* format string can result in strange
output because the errno value wasn't saved before calling other libc
functions. The solution is to simply save and restore the errno before
we format the user-supplied format string.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>