Ramdisk: add metadata dir in ramdisk

Add the matadata directory for the non-VAB device to eliminate the
following init error logs:
[    1.891172] init: [libfs_mgr]check_fs():
mount(/dev/block/by-name/metadata,/metadata,ext4)=-1: No such
file or directory

Test: dmesg | grep metadata | grep "No such file or directory"

Change-Id: Ie4db10bd3cbc194d05e64ffb954f1956db2321c6
Signed-off-by: lijiazi <lijiazi@xiaomi.com>