Revert "Using file** as fd table in proc struct, which fixes the fork bug in bionic"

This reverts commit a69eb385015e1284ef9993b22d1ec7430efee112 which lead
kernel to crash when invoking 'ls' on i386 and x86_64.

Conflicts:
	ucore/src/kern-ucore/fs/file.c
	ucore/src/kern-ucore/fs/file.h
	ucore/src/kern-ucore/fs/fs.c
	ucore/src/kern-ucore/fs/fs.h
	ucore/src/kern-ucore/mm/vmm.c

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>