nilfs2: fix missing initialization of i_dir_start_lookup member

The i_dir_start_lookup field in nilfs_inode_info objects should be
cleared when the objects are allocated, but the the initialization was
missing in case of reading from disk.  This adds the initialization.

Since the variable just gives a start page on directory lookups, the
bug was nonfatal until now.

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