libext2fs: Fix memory leak in the extents handling function

libext2fs: Fix memory leak in the extents handling function

Each time an extent handle is opened and closed, if the inode has an
extent tree which does not fit in the inode's i_block structure, a
filesystem block buffer was not getting released.  Since e2fsck opens
an extent handle for every inode using extents, this can translate to
a very large amount of memory getting lost.

Thanks to Henrik 'Mauritz' Johnson for discovering and pointing out
this leak, which he ran into while running the "rdump" command in
debugfs.

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