Don't bail out from the check if readboot() returns !FSFATAL.

This can happen when the fsinfo signature is invalid, and the
user have choose to fix it, in which case the code would return
FSBOOTMOD (not FSOK but not FSFATAL either).

All other (fatal) cases would return FSFATAL.

Obtained from: Android Open Source Project
Obtained from: https://android.googlesource.com/platform/external/fsck_msdos/+/d8775a29ea7eac2e5f1504dd21da3725b93b3036
MFC after: 2 weeks