checkpatch: attempt to find unnecessary 'out of memory' messages

Logging messages that show some type of "out of memory" error are
generally unnecessary as there is a generic message and a stack dump
done by the memory subsystem.

These messages generally increase kernel size without much added value.

Emit a warning on these types of messages.

This test looks for any inserted message function, then looks at the
previous line for an "if (!foo)" or "if (foo == NULL)" test and then
looks at the preceding statement for an allocation function like "foo =
kmalloc()"

ie: this code matches:

	foo = kmalloc();
	if (foo == NULL) {
		printk("Out of memory\n");
		return -ENOMEM;
	}

This test is very crude and incomplete.

This test can miss quite a lot of of OOM messages that do not have this
specific form.

ie: this code does not match:

	foo = kmalloc();
	if (!foo) {
		rtn = -ENOMEM;
		printk("Out of memory!\n");
		goto out;
	}

This test could also be a false positive when the logging message itself
does not specify anything about memory, but I did not find any false
positives in my limited testing.

spatch could be a better solution but correctness seems non-trivial for
that tool too.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>