selftests: prctl: Add prctl test for PR_GET_NAME

This patch covers the testing of PR_GET_NAME by
reading it's value from proc/self/task/pid/comm
and matching it with the value returned by PR_GET_NAME.
If the values are matched then it's successful, otherwise
it fails.

changes since v1:
	- Handled fscanf,fopen error checking.
	- Defined MAX_PATH_LEN.

Signed-off-by: Osama Muhammad <osmtendev@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>