staging: erofs: clean up internal.h

Tidy up relative order of variables / declarations in internal.h,
move some local static functions out into other files and
add tags at the end of #endif acrossing several lines.

No logic change.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-7-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>