fs_mgr: Use libdm for dm-linear devices.

This removes the custom ioctl code in fs_mgr_dm_linear.cpp in favor of
using libdm.

Bug: 110035986
Test: logical partitions are still created in first-stage init
Change-Id: I941a27ce865aa1bc712b91d8d4c874462e0d11df
3 files changed