add generic symlink to access userdata sysfs

This patch adds fixed location to access sysfs entries by introducing:
/dev/sys/fs/by-name/userdata -> /sys/fs/f2fs/dm-<num>
/dev/sys/block/by-name/userdata -> /sys/block/dm-<num>

Bug: 144277579
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: I1b3768005fc8720694bace05292a1b1171d60e3c