net: rds: fix rds_ib_sysctl_max_recv_allocation error

net: rds: fix rds_ib_sysctl_max_recv_allocation error

Before the commit c682e8474bd4 ("net/rds: reduce memory footprint
during ib_post_recv in IB transport"), rds_ib_allocation increases
by one. So the function atomic_add_unless will work. After the commit,
rds_ib_allocation increases by 4 if the frag is 16K. Then
atomic_add_unless will not work.

Fixes: c682e8474bd4 ("net/rds: reduce memory footprint during ib_post_recv in IB transport")
Orabug: 28947481

Change-Id: Ib032cd170d28e403a888c86124b67892b25ed5a5
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Reported-by: Joe Jin <joe.jin@oracle.com>
Reviewed-by: Joe Jin <joe.jin@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>