linux_compat: handle __GFP_ZERO in kmalloc()

Currently, kzalloc() returns zero-filled memory, while kmalloc()
simply ignores the second argument and never fills the memory
area with zeros.

I want kmalloc(size, __GFP_ZERO) to behave as kzalloc() does,
which will make it easier to add more memory allocator variants.

With the introduction of __GFP_ZERO flag, going forward, kzmalloc()
variants can fall back to kmalloc() enabling the __GFP_ZERO flag.

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