fs_mgr: use std::string for function parameters in fs_mgr_dm_ioctl.cpp

Make the code more C++-ish by replacing char* or char** with
std::string&.

Bug: 31264231
Test: check device can boot with dm-verity
Change-Id: Ie3ca3f449b52959d1a7b2887d722eb5ce366c4f9
3 files changed