tests: add new test f_zero_extent_length

tests: add new test f_zero_extent_length

If all of the extents in the last extent tree block (ETB) in a
non-trivial extent tree contain uninitialized extents which are after
the end of the file as defined by i_size, the hueristics will
incorrectly estimate the last entry (and hence the node's e_len field)
in the last entry of each level of the extent tree.

As Tao Ma has noted, since e2fsck was requiring that the length
(e_len) field of interior nodes be non-zero, this was causing false
failures where e2fsck would declare that the extent tree was
corrupted.

This was fixed in commit 9c40d14841: "e2fsck: only check for
zero-length leaf extents".  Add a regression test case to ensure that
this issue remains fixed.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>