iterate_and_advance(): get rid of magic in case when n is 0

iov_iter_advance() needs to do some non-trivial work when it's given
0 as argument (skip all empty iovecs, mostly).  We used to implement
it via iterate_and_advance(); we no longer do so and for all other
users of iterate_and_advance() zero length is a no-op.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -117,7 +117,7 @@
 #define iterate_and_advance(i, n, v, I, B, K, X) {		\
 	if (unlikely(i->count < n))				\
 		n = i->count;					\
-	if (i->count) {						\
+	if (likely(n)) {					\
 		size_t skip = i->iov_offset;			\
 		if (likely(iter_is_iovec(i))) {			\
 			const struct iovec *iov;		\