fs_mgr: moves common functions out of fs_mgr_verity.cpp

This commits moves some common functions out of fs_mgr_verity.cpp to be
reused by other verified boot flows. It includes:
 - Move common functions into fs_mgr.c
 - Move dm_ioctl related functions to a new file: fs_mgr_dm_ioctl.cpp

Bug: 31264231
Test: check device can boot with dm-verity

Change-Id: Iaa0d8031efbaae12aa28f872f62d3fc3d3763b51