nilfs2: fix missing block address termination in btree node shrinking

nilfs_btree_delete function does not terminate part of virtual block
addresses when shrinking the last remaining child node into the root
node.  The missing address termination causes that dead btree node
blocks persist and chip away free disk space.

This fixes the leak bug on the btree node deletion.

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