IPoIB: Fix race in deleting ipoib_neigh entries

In several places, this snippet is used when removing neigh entries:

	list_del(&neigh->list);
	ipoib_neigh_free(neigh);

The list_del() removes neigh from the associated struct ipoib_path, while
ipoib_neigh_free() removes neigh from the device's neigh entry lookup
table.  Both of these operations are protected by the priv->lock
spinlock.  The table however is also protected via RCU, and so naturally
the lock is not held when doing reads.

This leads to a race condition, in which a thread may successfully look
up a neigh entry that has already been deleted from neigh->list.  Since
the previous deletion will have marked the entry with poison, a second
list_del() on the object will cause a panic:

  #5 [ffff8802338c3c70] general_protection at ffffffff815108c5
     [exception RIP: list_del+16]
     RIP: ffffffff81289020  RSP: ffff8802338c3d20  RFLAGS: 00010082
     RAX: dead000000200200  RBX: ffff880433e60c88  RCX: 0000000000009e6c
     RDX: 0000000000000246  RSI: ffff8806012ca298  RDI: ffff880433e60c88
     RBP: ffff8802338c3d30   R8: ffff8806012ca2e8   R9: 00000000ffffffff
     R10: 0000000000000001  R11: 0000000000000000  R12: ffff8804346b2020
     R13: ffff88032a3e7540  R14: ffff8804346b26e0  R15: 0000000000000246
     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0000
  #6 [ffff8802338c3d38] ipoib_cm_tx_handler at ffffffffa066fe0a [ib_ipoib]
  #7 [ffff8802338c3d98] cm_process_work at ffffffffa05149a7 [ib_cm]
  #8 [ffff8802338c3de8] cm_work_handler at ffffffffa05161aa [ib_cm]
  #9 [ffff8802338c3e38] worker_thread at ffffffff81090e10
 #10 [ffff8802338c3ee8] kthread at ffffffff81096c66
 #11 [ffff8802338c3f48] kernel_thread at ffffffff8100c0ca

We move the list_del() into ipoib_neigh_free(), so that deletion happens
only once, after the entry has been successfully removed from the lookup
table.  This same behavior is already used in ipoib_del_neighs_by_gid()
and __ipoib_reap_neigh().

Signed-off-by: Jim Foraker <foraker1@llnl.gov>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Jack Wang <jinpu.wang@profitbricks.com>
Reviewed-by: Shlomo Pongratz <shlomop@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>