libdm: add an api to unwind dm stack

This API is an attempt to consolidate dm-stack unwinding logic
re-implemented in several different places (libfsmgr, libfstab,
libfiemap, etc.).

Test: libdm_test
Bug: 135984674
Change-Id: I9672d4c1e882824ef980667818d4c3aabcb82e1f