smb/server: use lookup_one_unlocked()

In process_query_dir_entries(), instead of locking the directory,
performing a lookup, then unlocking, we can simply call
lookup_one_unlocked().  That takes locks the directory only when needed.

This removes the only users of lock_dir() and unlock_dir() so they can
be removed.

Signed-off-by: NeilBrown <neil@brown.name>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>