CHROMIUM: scheduler: panic if schedulin while atomic

CHROMIUM: scheduler: panic if schedulin while atomic

When the schduling while atomic condition is hit, currently only a
printk and dump_stack happens but the kernel continues to run.  We've
run into issues where we think this causes a hang during shutdown
eventually.

Now doing a kernel panic instead by calling BUG().  This should prevent
hangs at shutdown from happening as well as providing us stack traces
that might help us root cause the real issue.

Signed-off-by: Sameer Nanda <snanda@chromium.org>
BUG=chromium-os:29599
TEST=We don't have a good way of repro'ing the hang on shutdown so the
testing is limited to kernel building fine.

Change-Id: I52084abf64a133a00f20e2eeaeb921820755e252
Reviewed-on: https://gerrit.chromium.org/gerrit/21534
Reviewed-by: Olof Johansson <olofj@chromium.org>
Commit-Ready: Sameer Nanda <snanda@chromium.org>
Tested-by: Sameer Nanda <snanda@chromium.org>