RDS: Use page_remainder_alloc() for recv bufs

Instead of splitting up a page into RDS_FRAG_SIZE chunks
ourselves, ask rds_page_remainder_alloc() to do it. While it
is possible PAGE_SIZE > FRAG_SIZE, on x86en it isn't, so having
duplicate "carve up a page into buffers" code seems excessive.

The other modification this spawns is the use of a single
struct scatterlist in rds_page_frag instead of a bare page ptr.
This causes verbosity to increase in some places, and decrease
in others.

Finally, I decided to unify the lifetimes and alloc/free of
rds_page_frag and its page. This is a nice simplification in itself,
but will be extra-nice once we come to adding cmason's recycling
patch.

Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/page.c

@@ -186,6 +186,7 @@ out:
 		 ret ? 0 : scat->length);
 	return ret;
 }
+EXPORT_SYMBOL_GPL(rds_page_remainder_alloc);
 
 static int rds_page_remainder_cpu_notify(struct notifier_block *self,
 					 unsigned long action, void *hcpu)