libext2fs: fix block iterator when the callback function modifies an extent

libext2fs: fix block iterator when the callback function modifies an extent

If the callback interator modifies a block in the middle of an extent
during a call to the block iterator, causing the extent to be split,
ext2_block_iterate3() will end up calling the callback function twice
for some number of blocks.  Fix this.

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