libext2fs: fix block iterator for extents when truncating inodes

libext2fs: fix block iterator for extents when truncating inodes

When e2fsck uses the block iterator to release the blocks in an
extent-mapped inode, when the last block in an extent is removed, the
current extent has been removed and the extent cursor is now pointing
at the next inode.  But the block iterator code doesn't know that.  So
when it tries to go the next extent, it will end up skipping an
extent, and so the inode will be incompletely truncated.

The fix is to go to the next extent before calling the callback
function for the current extent.  This way, regardless of whether the
current extent gets removed, the extent cursor is still pointing at
the right place.

Reported-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>