fs_mgr: code clean up

- Returns FS_MGR_MNTALL_FAIL for failure paths in fs_mgr_mount_all()
- Removes the 'goto out' in fs_mgr_do_mount() as there is nothing to do in
  the 'out' label now. Also removes the "ret = FS_MGR_DOMNT_FAILED;" and
  just return FS_MGR_DOMNT_FAILED directly for the default failure path.
- Changes some LERROR to PERROR

Test: Use fs_mgr_do_mount() to mount /system with AVB
Change-Id: I126a0124a5c9d61302f40ab9db16989500d9777e