aio_read_events_ring(): make a bit more readable

The logics for 'avail' is
	* not past the tail of cyclic buffer
	* no more than asked
	* not past the end of buffer
	* not past the end of a page

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

fs/aio.c

@@ -1197,14 +1197,13 @@ static long aio_read_events_ring(struct kioctx *ctx,
 		if (head == tail)
 			break;
 
-		avail = min(avail, nr - ret);
-		avail = min_t(long, avail, AIO_EVENTS_PER_PAGE -
-			    ((head + AIO_EVENTS_OFFSET) % AIO_EVENTS_PER_PAGE));
-
 		pos = head + AIO_EVENTS_OFFSET;
 		page = ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE];
 		pos %= AIO_EVENTS_PER_PAGE;
 
+		avail = min(avail, nr - ret);
+		avail = min_t(long, avail, AIO_EVENTS_PER_PAGE - pos);
+
 		ev = kmap(page);
 		copy_ret = copy_to_user(event + ret, ev + pos,
 					sizeof(*ev) * avail);