docbook: fix vmalloc missing parameter notation

Fix vmalloc kernel-doc warning:

Warning(linux-2.6.25-git14//mm/vmalloc.c:555): No description found for parameter 'caller'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>