initramfs: Support initrd that is bigger than 2GiB

When initrd (compressed or not) is used, kernel report data corrupted
with /dev/ram0.

The root cause:
During initramfs checking, if it is initrd, it will be transferred to
/initrd.image with sys_write.
sys_write only support 2G-4K write, so if the initrd ram is more than
that, /initrd.image will not complete at all.

Add local xwrite to loop calling sys_write to workaround the
problem.

Also need to use xwrite in write_buffer() to handle:
image is uncompressed cpio and there is one big file (>2G) in it.
   unpack_to_rootfs ===> write_buffer ===> actions[]/do_copy

At the same time, we don't need to worry about sys_read/sys_write in
do_mounts_rd.c::crd_load. As decompressor will have fill/flush and
local buffer that is smaller than 2G.

Test with uncompressed initrd, and compressed ones with gz, bz2, lzma,xz,
lzop.

-v2: according to HPA, change name to xwrite.
-v3: change xwrite to HPA's version. The normal behavior of xwrite(),
     like fwrite() and write(), is to return the total number of bytes
     written if any bytes are written at all.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: H. Peter Anvin <hpa@zytor.com>