kselftest: make ksft_exit_skip() output a reason for skipping

Make ksft_exit_skip() input an optional message string as the reason
for skipping all the tests and outputs it prior to exiting.

Signed-off-by: Paul Elder <paul.elder@pitt.edu>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>