libext2fs: reset handle after inserting new extent

libext2fs: reset handle after inserting new extent

Commit 53422e moved the new extent insertion in
ext2fs_extent_set_bmap() prior to the modification of the original
extent, but the insert function left the handle pointing to the new
extent.  This left us modifying the -new- extent, instead of the
original one, and winding up with a corrupt extent tree something
like:

BLOCKS:
(0-1):588791-588792, (0):588791

We need to move back to the previous extent prior
to modification, if we inserted a new one.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>