Prevent i_dtime from being mistaken for an inode number post-2038 wraparound

Prevent i_dtime from being mistaken for an inode number post-2038 wraparound

We explicitly decided not to reserve space for a 64-bit dtime, since
it's never displayed or exposed to userspace.  The dtime field is used
a linked list for the ophan list, and for forensic purposes when
trying to determine when an inode was deleted.  So right after the
2038 epoch, a deleted inode might end up with a dtime which is zero or
smaller than the number of inodes, which will result in e2fsck
reporting a potential problems.  So when we set the dtime, make sure
that the dtime won't be mistaken for an inode number.

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