3.14-rc2 XFS backtrace because irqs_disabled.

[Al Viro]

On Tue, Feb 11, 2014 at 07:44:03PM -0500, Dave Jones wrote:

> The 'good' news is I seem to be able to reproduce it fairly quickly.
> I've seen it 3-4 times this afternoon. (Always from the coredump path)
> 
> I was on vacation last week, but I wasn't hitting this before then, so it's a
> fairly recent change that's introduced this.

Slap the check in vfs_create(), see if interrupts had been disabled by it or
by something in ->create().  Since it's reproducible...