fs_mgr: add sysfs_path=%s to indicate a location of sysfs entry

This patch adds sysfs_path=%s in fstab to get the sysfs entry path.
One example is that vold is able to conduct UFS-specific operations given the
below fstab entry.

/dev/block/bootdevice/by-name/userdata  /data f2fs  noatime,nosuid,nodev,discard
  latemount,wait,check,fileencryption=ice,quota,formattable,sysfs_path=/sys/devices/platform/soc/1d84000.ufshc

Bug: 63264275
Bug: 68721792
Change-Id: I5097873893948f12296ac5eaa0c2a59a6e6dd9e1
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>