net/rds: Fix NULL/ERR_PTR inconsistency

Make function "rds_ib_try_reuse_ibmr" return NULL in case
memory region could not be allocated, since callers
simply check if the return value is not NULL.

Signed-off-by: Gerd Rausch <gerd.rausch@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>