fix O_SYNC|O_APPEND syncing the wrong range on write()

It actually goes back to 2004 ([PATCH] Concurrent O_SYNC write support)
when sync_page_range() had been introduced; generic_file_write{,v}() correctly
synced
	pos_after_write - written .. pos_after_write - 1
but generic_file_aio_write() synced
	pos_before_write .. pos_before_write + written - 1
instead.  Which is not the same thing with O_APPEND, obviously.
A couple of years later correct variant had been killed off when
everything switched to use of generic_file_aio_write().

All users of generic_file_aio_write() are affected, and the same bug
has been copied into other instances of ->aio_write().

The fix is trivial; the only subtle point is that generic_write_sync()
ought to be inlined to avoid calculations useless for the majority of
calls.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/cifs/file.c

@@ -2559,8 +2559,8 @@ cifs_writev(struct kiocb *iocb, const struct iovec *iov,
 	if (rc > 0) {
 		ssize_t err;
 
-		err = generic_write_sync(file, pos, rc);
-		if (err < 0 && rc > 0)
+		err = generic_write_sync(file, iocb->ki_pos - rc, rc);
+		if (err < 0)
 			rc = err;
 	}