checkpatch: add exception to return then else test

Add an exception to the return before else warning when the line
following it is also a return like:

	if (foo)
		return bar;
	else
		return baz;

This form of a test then return is at least as readable as

	if (foo)
		return bar;
	return baz;

so don't emit a warning on the first form.

Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Elshad Mustafayev <elshadimo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>