nvme-rdma: use dynamic dma mapping per command

Commit 87fd1253 ("nvme-rdma: remove redundant reference between
ib_device and tagset") caused a kernel panic when disconnecting from an
inaccessible controller (disconnect during re-connection).

--
nvme nvme0: Removing ctrl: NQN "testnqn1"
nvme_rdma: nvme_rdma_exit_request: hctx 0 queue_idx 1
BUG: unable to handle kernel paging request at 0000000080000228
PGD 0 P4D 0
Oops: 0000 [#1] SMP PTI
...
Call Trace:
 blk_mq_exit_hctx+0x5c/0xf0
 blk_mq_exit_queue+0xd4/0x100
 blk_cleanup_queue+0x9a/0xc0
 nvme_rdma_destroy_io_queues+0x52/0x60 [nvme_rdma]
 nvme_rdma_shutdown_ctrl+0x3e/0x80 [nvme_rdma]
 nvme_do_delete_ctrl+0x53/0x80 [nvme_core]
 nvme_sysfs_delete+0x45/0x60 [nvme_core]
 kernfs_fop_write+0x105/0x180
 vfs_write+0xad/0x1a0
 ksys_write+0x5a/0xd0
 do_syscall_64+0x55/0x110
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7fa215417154
--

The reason for this crash is accessing an already freed ib_device for
performing dma_unmap during exit_request commands. The root cause for
that is that during re-connection all the queues are destroyed and
re-created (and the ib_device is reference counted by the queues and
freed as well) but the tagset stays alive and all the DMA mappings (that
we perform in init_request) kept in the request context. The original
commit fixed a different bug that was introduced during bonding (aka nic
teaming) tests that for some scenarios change the underlying ib_device
and caused memory leakage and possible segmentation fault. This commit
is a complementary commit that also changes the wrong DMA mappings that
were saved in the request context and making the request sqe dma
mappings dynamic with the command lifetime (i.e. mapped in .queue_rq and
unmapped in .complete). It also fixes the above crash of accessing freed
ib_device during destruction of the tagset.

Fixes: 87fd1253 ("nvme-rdma: remove redundant reference between ib_device and tagset")
Reported-by: Jim Harris <james.r.harris@intel.com>
Suggested-by: Sagi Grimberg <sagi@grimberg.me>
Tested-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>