e2fsck: make the low dtime check consistent when using the backup superblock

e2fsck: make the low dtime check consistent when using the backup superblock

The backup superblock may have a last mounted time of zero, if it has
never been updated since the file system was created.  In that case,
the low dtime check may get disabled when using the backup superblock,
even though subsequent e2fsck runs will end up using the low dtime
check.  This can cause a failure of ext4/007, since since when e2fsck
is run a second time after the file system is mounted, the low dtime
check will trigger the e2fsck complaint:

Inode NNNN was part of the orphaned inode list.  IGNORED.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>