libext2fs: fix i_blocks for extent leaf create/delete with bigalloc

libext2fs: fix i_blocks for extent leaf create/delete with bigalloc

When libext2fs allocates/deletes an extent leaf, the i_blocks
value is incremented/decremented by fs->blocksize / 512. This
is incorrect in case of bigalloc. The correct way here is to
use cluster_size / 512.

The problem is seen if we try to create a large inode using
libext2fs (say using ext2fs_block_iterate3()) on a bigalloc
filesystem. fsck catches this and complains.

Signed-off-by: Aditya Kali <adityakali@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>