3.14-rc2 XFS backtrace because irqs_disabled.

[Al Viro]

On Tue, Feb 11, 2014 at 10:28:12PM -0800, Linus Torvalds wrote:

> It looks like just "do_signal()" has a stack frame that is about 230
> bytes even under normal circumstancs (largely due to "struct ksignal"
> - which in turn is largely due to the insane 128-byte padding in
> siginfo_t). Add a few other frames in there, and I guess that if it
> was close before, the coredump path just makes it go off.

We could, in principle, put it into task_struct and make get_signal()
return its address - do_signal() is called only in the code that does
assorted returns to userland...