fs_mgr_get_mounted_entry_for_userdata: handle logical partitions

In case of GSI on DSU, /data is mounted on the following stack:
userdata -> userdata_gsi -> actual block device, and fstab returned from
ReadDefaultFstab() function will say that /data should be
mounted onto userdata_gsi. This configuration wasn't supported.

To support it, fs_mgr_get_mounted_entry_for_userdata now unwinds the
dm-device stack /data is mounted onto, and for each entry in fstab
resolves corresponding block device (in case of logical partitions by
asking DeviceMapper to resolve name -> path, and for other cases by
reading a symlink), and then checking whenever resolved block device
belongs to the unwinded stack.

Test: on both normal boot & DSU
Test: atest CtsUserspaceRebootHostSideTestCases
Test: atest CtsFsMgrTestCases
Bug: 148612148
Change-Id: I1bb3e64ae92d56de905b4f7f1997b7499b101b0f