libfiemap_writer: Improve device unwrapping to support simple linear devices.

Normally we reject complex dm targets that would invalidate or shift the
block mappings returned via FIEMAP/FIBMAP. Currently the only targets
allowed are crypt, default-key, and bow. This patch adds support for
"linear" as long as there is only one linear target and it targets
sector 0 of the underlying block device.

This is useful for testing gsid, so we can simulate how a
metadata-encrypted device works without having to create a dm-crypt or
dm-default-key node.

Bug: 134536978
Test: manual test
Change-Id: I7c12bc20d95ff4c90402e66bafb4cf2fce7818e2