linux_compat: move vzalloc() to header file as an inline function

The vzalloc(size) is equivalent to kzalloc(size, 0).  Move it to
include/linux/compat.h as an inline function in order to avoid the
function call overhead.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>