CodingStyle: add some more error handling guidelines

I added a paragraph on choosing label names, and updated the example
code to use a better label name.  I also cleaned up the example code to
more modern style by moving the allocation out of the initializer and
changing the NULL check.

Perhaps the most common type of error handling bug in the kernel is "one
err bugs".  CodingStyle already says that we should "avoid nesting" by
using error labels and one err style error handling tends to have
multiple indent levels, so this was already bad style.  But I've added a
new paragraph explaining how to avoid one err bugs by using multiple
error labels which is, hopefully, more clear.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
[jc: added GFP_KERNEL to kmalloc() call]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>