net/smc: check for missing nlattrs in SMC_PNETID messages

It's possible to crash the kernel in several different ways by sending
messages to the SMC_PNETID generic netlink family that are missing the
expected attributes:

- Missing SMC_PNETID_NAME => null pointer dereference when comparing
  names.
- Missing SMC_PNETID_ETHNAME => null pointer dereference accessing
  smc_pnetentry::ndev.
- Missing SMC_PNETID_IBNAME => null pointer dereference accessing
  smc_pnetentry::smcibdev.
- Missing SMC_PNETID_IBPORT => out of bounds array access to
  smc_ib_device::pattr[-1].

Fix it by validating that all expected attributes are present and that
SMC_PNETID_IBPORT is nonzero.

Reported-by:  <syzbot+5cd61039dc9b8bfa6e47@syzkaller.appspotmail.com>
Fixes: 6812baab ("smc: establish pnet table management")
Cc: <stable@vger.kernel.org> # v4.11+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>