fs: ext2: Fix removing indirect blocks

This commit fixes removing indirect blocks (marking them as 0)
in the inode structure. Previous version of the code was removing
the top-level blocks only when the first removed block was
one of the first 12 direct blocks. However, when the first removed
block is the first block in the referenced block list, its parent
(indirect block) should also be removed.

Signed-off-by: Franciszek Pindel <fpindel@antmicro.com>