optimize copy_page_{to,from}_iter()

if we'd ended up in the end of a segment, jump to the
beginning of the next one (iov_offset = 0, iov++),
rather than having the next primitive deal with that.

Ought to be folded back...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/iov_iter.c

@@ -74,6 +74,10 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 	}
 	kunmap(page);
 done:
+	if (skip == iov->iov_len) {
+		iov++;
+		skip = 0;
+	}
 	i->count -= wanted - bytes;
 	i->nr_segs -= iov - i->iov;
 	i->iov = iov;
@@ -152,6 +156,10 @@ size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
 	}
 	kunmap(page);
 done:
+	if (skip == iov->iov_len) {
+		iov++;
+		skip = 0;
+	}
 	i->count -= wanted - bytes;
 	i->nr_segs -= iov - i->iov;
 	i->iov = iov;