RDMA/srpt: Fix TPG creation

RDMA/srpt: Fix TPG creation

Unlike the iSCSI target driver, for the SRP target driver it is sufficient
if a single TPG can be associated with each RDMA port name. However, users
started associating multiple TPGs with RDMA port names. Support this by
converting the single TPG in struct srpt_port_id into a list. This patch
fixes the following list corruption issue:

 list_add corruption. prev->next should be next (ffffffffc0a080c0), but was ffffa08a994ce6f0. (prev=ffffa08a994ce6f0).
 WARNING: CPU: 2 PID: 2597 at lib/list_debug.c:28 __list_add_valid+0x6a/0x70
 CPU: 2 PID: 2597 Comm: targetcli Not tainted 5.4.0-rc1.3bfa3c9602a7 #1
 RIP: 0010:__list_add_valid+0x6a/0x70
 Call Trace:
  core_tpg_register+0x116/0x200 [target_core_mod]
  srpt_make_tpg+0x3f/0x60 [ib_srpt]
  target_fabric_make_tpg+0x41/0x290 [target_core_mod]
  configfs_mkdir+0x158/0x3e0
  vfs_mkdir+0x108/0x1a0
  do_mkdirat+0x77/0xe0
  do_syscall_64+0x55/0x1d0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Link: https://lore.kernel.org/r/20191023204106.23326-1-bvanassche@acm.org
Reported-by: Honggang LI <honli@redhat.com>
Fixes: a42d985bd5b2 ("ib_srpt: Initial SRP Target merge for v3.3-rc1")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Honggang Li <honli@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>