tools/testing/selftests/ipc/msgque.c: improve error handling when not running as root

The test fails in the middle when it is not run as root while accessing
/proc/sys/kernel/msg_next_id.  Changed it to check for root at the
beginning of the test and exit if not root.

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>