fix __swap_writepage() compile failure on old gcc versions

Tetsuo Handa wrote:
 "Commit 62a8067a ("bio_vec-backed iov_iter") introduced an unnamed
  union inside a struct which gcc-4.4.7 cannot handle.  Name the unnamed
   union as u in order to fix build failure"

Let's do this instead: there is only one place in the entire tree that
steps into this breakage.  Anon structs and unions work in older gcc
versions; as the matter of fact, we have those in the tree - see e.g.
struct ieee80211_tx_info in include/net/mac80211.h

What doesn't work is handling their initializers:

struct {
	int a;
	union {
		int b;
		char c;
	};
} x[2] = {{.a = 1, .c = 'a'}, {.a = 0, .b = 1}};

is the obvious syntax for initializer, perfectly fine for C11 and
handled correctly by gcc-4.7 or later.

Earlier versions, though, break on it - declaration is fine and so's
access to fields (i.e.  x[0].c = 'a'; would produce the right code), but
members of the anon structs and unions are not inserted into the right
namespace.  Telling...

mm/page_io.c

@@ -274,8 +274,8 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 			.count = PAGE_SIZE,
 			.iov_offset = 0,
 			.nr_segs = 1,
-			.bvec = &bv
 		};
+		from.bvec = &bv;	/* older gcc versions are broken */
 
 		init_sync_kiocb(&kiocb, swap_file);
 		kiocb.ki_pos = page_file_offset(page);