RDMA/cma: Allow UD qp_type to join multicast only

In the Linux kernel, the following vulnerability has been resolved:

RDMA/cma: Allow UD qp_type to join multicast only

As for multicast:
- The SIDR is the only mode that makes sense;
- Besides PS_UDP, other port spaces like PS_IB is also allowed, as it is
UD compatible. In this case qkey also needs to be set [1].

This patch allows only UD qp_type to join multicast, and set qkey to
default if it's not set, to fix an uninit-value error: the ib->rec.qkey
field is accessed without being initialized.

=====================================================
BUG: KMSAN: uninit-value in cma_set_qkey drivers/infiniband/core/cma.c:510 [inline]
BUG: KMSAN: uninit-value in cma_make_mc_event+0xb73/0xe00 drivers/infiniband/core/cma.c:4570
cma_set_qkey drivers/infiniband/core/cma.c:510 [inline]
cma_make_mc_event+0xb73/0xe00 drivers/infiniband/core/cma.c:4570
cma_iboe_join_multicast drivers/infiniband/core/cma.c:4782 [inline]
rdma_join_multicast+0x2b83/0x30a0 drivers/infiniband/core/cma.c:4814
ucma_process_join+0xa76/0xf60 drivers/infiniband/core/ucma.c:1479
ucma_join_multicast+0x1e3/0x250 drivers/infiniband/core/ucma.c:1546
ucma_write+0x639/0x6d0 drivers/infiniband/core/ucma.c:1732
vfs_write+0x8ce/0x2030 fs/read_write.c:588
ksys_write+0x28c/0x520 fs/read_write.c:643
__do_sys_write fs/read_write.c:655 [inline]
__se_sys_write fs/read_write.c:652 [inline]
__ia32_sys_write+0xdb/0x120 fs/read_write.c:652
do_syscall_32_irqs_on arch/x86/entry/common.c:114 [inline]
__do_fast_syscall_32+0x96/0xf0 arch/x86/entry/common.c:180
do_fast_syscall_32+0x34/0x70 arch/x86/entry/common.c:205
do_SYSENTER_32+0x1b/0x20 arch/x86/entry/common.c:248
entry_SYSENTER_compat_after_hwframe+0x4d/0x5c

Local variable ib.i created at:
cma_iboe_join_multicast drivers/infiniband/core/cma.c:4737 [inline]
rdma_join_multicast+0x586/0x30a0 drivers/infiniband/core/cma.c:4814
ucma_process_join+0xa76/0xf60 drivers/infiniband/core/ucma.c:1479

CPU: 0 PID: 29874 Comm: syz-executor.3 Not tainted 5.16.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
=====================================================

[1] https://lore.kernel.org/linux-rdma/20220117183832.GD84788@nvidia.com/