sched: reset sched_entity depth on changing parent

From 2014-02-15: https://lkml.org/lkml/2014/2/15/217

This issue was reported and patched, but it still occurs in some situations on
newer kernel versions.

[2249353.328452] BUG: unable to handle kernel NULL pointer dereference at 0000000000000150
[2249353.336528] IP: [<ffffffff810b1cf7>] check_preempt_wakeup+0xe7/0x210

se.parent gets out of sync with se.depth, causing a panic when the algorithm in
find_matching_se assumes they are correct. This patch forces se.depth to be
updated every time se.parent is, so they can no longer become desync'd.

CC: Ingo Molnar <mingo@kernel.org>
CC: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Burke Libbey <burke.libbey@shopify.com>