e2fsck: offer to clear overlapping extents

e2fsck: offer to clear overlapping extents

If in the course of iterating extents we find that an otherwise
valid-seeming second extent maps the same logical blocks as a
previously examined first extent, offer to clear the duplicate
mapping.

The test for this is already in f_extents.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>