cachefiles: only pass inode to *mark_inode_inuse() helpers

The only reason to pass dentry was because of a pr_notice() text.  Move
that to the two callers where it makes sense and add a WARN_ON() to the
third.

file_inode(file) is never NULL on an opened file. Remove check in
cachefiles_unmark_inode_in_use().

Do not open code cachefiles_do_unmark_inode_in_use() in
cachefiles_put_directory().

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>