fast_dput(): new rules for refcount

By now there is only one place in entire fast_dput() where we return
false; that happens after refcount had been decremented and found (under
->d_lock) to be zero.  In that case, just prior to returning false to
caller, fast_dput() forcibly changes the refcount from 0 to 1.

Lift that resetting refcount to 1 into the callers; later in the series
it will be massaged out of existence.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>