libext2fs: fix punching extents when there are no left extents

libext2fs: fix punching extents when there are no left extents

When deleting an entire extent, we cannot always slip to the previous
leaf extent because there might not /be/ a previous extent.
Attempting to correct for that error by asking for the 'current' leaf
extent also doesn't work, because the failed attempt to change to the
previous extent leaves us with no current extent.

Fix this problem by recording the lblk of the next extent before
deleting the current extent and _goto()ing to the next extent after
the deletion.

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