prctl: adds the capable(CAP_SYS_NICE) check to PR_SET_TIMERSLACK_PID.

Adds a capable() check to make sure that arbitary apps do not change the
timer slack for other apps.

Bug: 15000427
Change-Id: I558a2551a0e3579c7f7e7aae54b28aa9d982b209
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>
Git-commit: 8ae872f1d59990a1f33a7e5aae6b5bcbedc5806f
Git-repo: https://android.googlesource.com/kernel/common.git


Signed-off-by: Ian Maund <imaund@codeaurora.org>