checkpatch: add checks for in_atomic()

in_atomic() is not for driver use so report any such use as an ERROR.
Also in_atomic() is often used to determine if we may sleep, but it is not
reliable in this use model therefore strongly discourage its use.

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