check ext4 magic before running next steps

- mount, e2fsck, tune2fs will all fail if magic number does not match.
- mismatch always happen for FDE and is wasting boot-up time to try
  all and fail always.
- skip mount steps if it has invalid magic number and do not record
  fs_stat either.
- For ext4 fs with corrupt superblock, e2fsck refuses to do anything if
  superblock magic is invalid. So simply running e2fsck does not help
  anyway.

bug: 36231950
Test: reboot ane check fs_mgr log from dmesg

(cherry picked from commit 6000a3f6)

Change-Id: Ia7120a188c316262da5fdb986c7d9c76db86aa7b