nilfs2: fix incorrect block address termination in node concatenation

nilfs_btree_delete function wrongly terminates virtual block address
of the btree node held by its parent at index 0.  When concatenating
the index-0 node with its right sibling node, nilfs_btree_delete
terminates the block address of index-0 node instead of the right
sibling node which should be deleted.

This bug not only wears disk space in the long run, but also causes
file system corruption.  This will fix it.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>