e2fsck: fix use after free in calculate_tree()

e2fsck: fix use after free in calculate_tree()

The problem is alloc_blocks() will call get_next_block() which might
reallocate outdir->buf, and memory address could be changed after
this.  To fix this, pointers that point into outdir->buf, such as
int_limit and root need to be recaulated based on the new starting
address of outdir->buf.

[ Changed to correctly recalculate int_limit, and to optimize how we
  reallocate outdir->buf.  -TYT ]

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>