init/fs_mgr: prototype first-stage dm-linear support

This adds an API to fs_mgr for reading dm-linear tables out of device trees
and issuing device-mapper ioctls. The device tree code will be
implemented separately. The dm-linear structures in fs_mgr are organized
assuming we may want to pull them from sources other than DT (for
example, text files, binary blobs, or something hardcoded for testing).

File systems which are mounted from these logical partitions have
specific fstab requirements. The block device must be a partition name,
and if Verified Boot is used, that name must match the vbmeta partition name.
Second, the entry must have the "logical" fs_mgr flag. Example fstab

    vendor  /vendor  ext4  ro  wait,logical

Example fstab entry in device tree:

    vendor {
        compatible = "android,fstab";
        dev = "vendor";
        type = "ext4";
        mnt_flags = "ro";
        fs_mgr_flags = "wait,slotselect,avb,logical";

Bug: 78914864
Test: N/A
Change-Id: I4d8878ea8858f26310119616cadc3ee0dd08566c
11 files changed