checkpatch: add a checkpatch warning for new uses of __initcall().

[apw@shadowen.org: generalise pattern and add tests]
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
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>