[PATCH] Implement kasprintf

Implement kasprintf, a kernel version of asprintf.  This allocates the
memory required for the formatted string, including the trailing '\0'.
Returns NULL on allocation failure.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>