fuse2fs: spot check clean journals

fuse2fs: spot check clean journals

Even though fuse2fs doesn't (yet) support writing new transactions to
the journal, we ought at least to check that the superblock is ok when
we mount a clean filesystem.  This fixes complaints by ext4/012 about
mount failing to notice a corrupt journal.

Cc: <linux-ext4@vger.kernel.org> # v1.43
Fixes: 81cbf1ef4f5dab ("misc: add fuse2fs, a FUSE server for e2fsprogs")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>