checkpatch: make in_atomic ok in the core

We say that in_atomic() is ok in the core kernel, but then always report
it regardless of where in the kernel it is.  Keep quiet if it is used in
kernel/*.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>