libext2fs: rewind extent pointer when totally deleting an extent

libext2fs: rewind extent pointer when totally deleting an extent

During a punch operation, if we decide to delete an extent out of the
extent tree, the subsequent extents are moved on top of the current
extent (that is to say, they're memmmove'd down one slot).  Therefore
it is not correct to advance to the next leaf because that means we
miss half the extents in the range!  Rereading the current pointer
should be fine.

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