switch ll_lookup_finish_locks() and ll_revalidate_it_finish() to inode

Note that ll_prep_inode() in the latter does *not* modify ->d_inode;
it expects non-negative dentry, and in such cases ll_prep_inode() doesn't
modify *inode - it only uses the value.

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