fs_mgr: Support checkpoints

Adds support for partitions with checkpointing enabled. If the
checkpoint= fs_mgr flag is set, and the system has checkpointing on,
the partition will be mounted in checkpointing mode.

Test: Use vdc checkpoint commands, the checkpoint=fs fs_mgr flag
      in the fstab, and a kernel containing the f2fs checkpoint
      changes. https://lkml.org/lkml/2018/8/21/22
Change-Id: I3ea8da932de06fcfd2eed06b8640a8b1df837f1f