Documentation: update CodingStyle memory allocators

The list of available general purpose memory allocators in
Documentation/CodingStyle chapter 14 is incomplete. This patch adds
the missing vzalloc() to the list.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>