RDMA/ucma: Don't allow join attempts for unsupported AF family

Users can provide garbage while calling to ucma_join_ip_multicast(),
it will indirectly cause to rdma_addr_size() return 0, making the
call to ucma_process_join(), which had the right checks, but it is
better to check the input as early as possible.

The following crash from syzkaller revealed it.

kernel BUG at lib/string.c:1052!
invalid opcode: 0000 [#1] SMP KASAN Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 4113 Comm: syz-executor0 Not tainted 4.16.0-rc5+ #261
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:fortify_panic+0x13/0x20 lib/string.c:1051
RSP: 0018:ffff8801ca81f8f0 EFLAGS: 00010286
RAX: 0000000000000022 RBX: 1ffff10039503f23 RCX: 0000000000000000
RDX: 0000000000000022 RSI: 1ffff10039503ed3 RDI: ffffed0039503f12
RBP: ffff8801ca81f8f0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000006 R11: 0000000000000000 R12: ffff8801ca81f998
R13: ffff8801ca81f938 R14: ffff8801ca81fa58 R15: 000000000000fa00
FS:  0000000000000000(0000) GS:ffff8801db200000(0063) knlGS:000000000a12a900
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000008138024 CR3: 00000001cbb58004 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 memcpy include/linux/string.h:344 [inline]
 ucma_join_ip_multicast+0x36b/0x3b0 drivers/infiniband/core/ucma.c:1421
 ucma_write+0x2d6/0x3d0 drivers/infiniband/core/ucma.c:1633
 __vfs_write+0xef/0x970 fs/read_write.c:480
 vfs_write+0x189/0x510 fs/read_write.c:544
 SYSC_write fs/read_write.c:589 [inline]
 SyS_write+0xef/0x220 fs/read_write.c:581
 do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
 do_fast_syscall_32+0x3ec/0xf9f arch/x86/entry/common.c:392
 entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7f9ec99
RSP: 002b:00000000ff8172cc EFLAGS: 00000282 ORIG_RAX: 0000000000000004
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000020000100
RDX: 0000000000000063 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
Code: 08 5b 41 5c 41 5d 41 5e 41 5f 5d c3 0f 0b 48 89 df e8 42 2c e3 fb eb de
55 48 89 fe 48 c7 c7 80 75 98 86 48 89 e5 e8 85 95 94 fb <0f> 0b 90 90 90 90
90 90 90 90 90 90 90 55 48 89 e5 41 57 41 56
RIP: fortify_panic+0x13/0x20 lib/string.c:1051 RSP: ffff8801ca81f8f0

Fixes: 5bc2b7b3 ("RDMA/ucma: Allow user space to specify AF_IB when joining multicast")
Reported-by:  <syzbot+2287ac532caa81900a4e@syzkaller.appspotmail.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>