nilfs_attach_log_writer(): use ->bd_mapping->host instead of ->bd_inode

I suspect that inode_attach_wb() use is rather unidiomatic, but
that's a separate story - in any case, its use is a few times
per mount *and* the route by which we access that inode is
"the host of address_space a page belongs to".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>