rds: fix crash in rds_info_getsockopt()

The conversion to pin_user_pages() had a bug: it overlooked
the case of allocation of pages failing. Fix that by restoring
an equivalent check.

Reported-by:  <syzbot+118ac0af4ac7f785a45b@syzkaller.appspotmail.com>
Fixes: dbfe7d74

 ("rds: convert get_user_pages() --> pin_user_pages()")

Cc: David S. Miller <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Cc: linux-rdma@vger.kernel.org
Cc: rds-devel@oss.oracle.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>