RDS: Fix assertion level from fatal to warning

Fix the asserion level since its not fatal and can be hit
in normal execution paths. There is no need to take the
system down.

We keep the WARN_ON() to detect the condition if we get
here with bad pages.

Reviewed-by: Ajaykumar Hotchandani <ajaykumar.hotchandani@oracle.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>