Empty device names cannot be added to sysfs and crash with:

  kobject: (00000000f9de3792): attempted to be registered with empty name!
  WARNING: CPU: 1 PID: 10856 at lib/kobject.c:234 kobject_add_internal+0x7ac/0x9a0 lib/kobject.c:234
  Kernel panic - not syncing: panic_on_warn set ...
  CPU: 1 PID: 10856 Comm: syz-executor459 Not tainted 5.6.0-rc3-syzkaller #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  Call Trace:
   __dump_stack lib/dump_stack.c:77 [inline]
   dump_stack+0x197/0x210 lib/dump_stack.c:118
   panic+0x2e3/0x75c kernel/panic.c:221
   __warn.cold+0x2f/0x3e kernel/panic.c:582
   report_bug+0x289/0x300 lib/bug.c:195
   fixup_bug arch/x86/kernel/traps.c:174 [inline]
   fixup_bug arch/x86/kernel/traps.c:169 [inline]
   do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:267
   do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:286
   invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
  RIP: 0010:kobject_add_internal+0x7ac/0x9a0 lib/kobject.c:234
  Code: 7a ca ca f9 e9 f0 f8 ff ff 4c 89 f7 e8 cd ca ca f9 e9 95 f9 ff ff e8 13 25 8c f9 4c 89 e6 48 c7 c7 a0 08 1a 89 e8 a3 76 5c f9 <0f> 0b 41 bd ea ff ff ff e9 52 ff ff ff e8 f2 24 8c f9 0f 0b e8 eb
  RSP: 0018:ffffc90002006eb0 EFLAGS: 00010286
  RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: ffffffff815eae46 RDI: fffff52000400dc8
  RBP: ffffc90002006f08 R08: ffff8880972ac500 R09: ffffed1015d26659
  R10: ffffed1015d26658 R11: ffff8880ae9332c7 R12: ffff888093034668
  R13: 0000000000000000 R14: ffff8880a69d7600 R15: 0000000000000001
   kobject_add_varg lib/kobject.c:390 [inline]
   kobject_add+0x150/0x1c0 lib/kobject.c:442
   device_add+0x3be/0x1d00 drivers/base/core.c:2412
   ib_register_device drivers/infiniband/core/device.c:1371 [inline]
   ib_register_device+0x93e/0xe40 drivers/infiniband/core/device.c:1343
   rxe_register_device+0x52e/0x655 drivers/infiniband/sw/rxe/rxe_verbs.c:1231
   rxe_add+0x122b/0x1661 drivers/infiniband/sw/rxe/rxe.c:302
   rxe_net_add+0x91/0xf0 drivers/infiniband/sw/rxe/rxe_net.c:539
   rxe_newlink+0x39/0x90 drivers/infiniband/sw/rxe/rxe.c:318
   nldev_newlink+0x28a/0x430 drivers/infiniband/core/nldev.c:1538
   rdma_nl_rcv_msg drivers/infiniband/core/netlink.c:195 [inline]
   rdma_nl_rcv_skb drivers/infiniband/core/netlink.c:239 [inline]
   rdma_nl_rcv+0x5d9/0x980 drivers/infiniband/core/netlink.c:259
   netlink_unicast_kernel net/netlink/af_netlink.c:1303 [inline]
   netlink_unicast+0x59e/0x7e0 net/netlink/af_netlink.c:1329
   netlink_sendmsg+0x91c/0xea0 net/netlink/af_netlink.c:1918
   sock_sendmsg_nosec net/socket.c:652 [inline]
   sock_sendmsg+0xd7/0x130 net/socket.c:672
   ____sys_sendmsg+0x753/0x880 net/socket.c:2343
   ___sys_sendmsg+0x100/0x170 net/socket.c:2397
   __sys_sendmsg+0x105/0x1d0 net/socket.c:2430
   __do_sys_sendmsg net/socket.c:2439 [inline]
   __se_sys_sendmsg net/socket.c:2437 [inline]
   __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2437
   do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

Prevent empty names when checking the name provided from userspace during
newlink and rename.

Fixes: 3856ec4b ("RDMA/core: Add RDMA_NLDEV_CMD_NEWLINK/DELLINK support")
Fixes: 05d940d3 ("RDMA/nldev: Allow IB device rename through RDMA netlink")
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20200309191648.GA30852@ziepe.ca
Reported-and-tested-by: syzbot+da615ac67d4dbea32cbc@syzkaller.appspotmail.com
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>