System crash in blk_mq_free_request() following NVMe controller reset due to NULL mq_hctx

Issue

• System crash in blk_mq_free_request() following NVMe controller reset due to NULL mq_hctx. Example 1:

nvme nvme0: I/O 71 QID 1 timeout, aborting
nvme nvme0: Abort status: 0x0
nvme nvme0: I/O 71 QID 1 timeout, reset controller
nvme nvme0: 2/0/0 default/read/poll queues
BUG: unable to handle kernel NULL pointer dereference at 0000000000000108
PGD 0 P4D 0 
Oops: 0000 [#1] SMP PTI
CPU: 0 PID: 0 Comm: swapper/0 Kdump: loaded Not tainted 4.18.0-348.2.1.el8_5.x86_64 #1
Hardware name: VMware, Inc. VMware7,1/440BX Desktop Reference Platform, BIOS VMW71.00V.18227214.B64.2106252220 06/25/2021
RIP: 0010:blk_mq_free_request+0xbc/0x120
....
Call Trace:
<IRQ>
nvme_irq+0x127/0x240 [nvme]
__handle_irq_event_percpu+0x40/0x180
handle_irq_event_percpu+0x30/0x80
handle_irq_event+0x36/0x53
handle_edge_irq+0x82/0x190
handle_irq+0x1c/0x30
do_IRQ+0x49/0xd0
common_interrupt+0xf/0xf
....

• Example 2:

nvme nvme0: I/O 197 QID 1 timeout, aborting
nvme nvme0: Abort status: 0x0
nvme nvme0: I/O 197 QID 1 timeout, reset controller
nvme nvme0: 4/0/0 default/read/poll queues
nvme nvme0: I/O 81 QID 2 timeout, aborting
nvme nvme0: Abort status: 0x0
nvme nvme0: I/O 81 QID 2 timeout, reset controller
nvme nvme0: 4/0/0 default/read/poll queues
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD 0 P4D 0 
Oops: 0002 [#1] SMP NOPTI
CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: G           OE    --------- -  - 4.18.0-193.el8.x86_64 #1
Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 12/12/2018
RIP: 0010:dma_pool_free+0x92/0xb0
....
Call Trace:
<IRQ>
nvme_unmap_data+0x113/0x1f0 [nvme]
nvme_pci_complete_rq+0x30/0x40 [nvme]
blk_mq_complete_request+0x6c/0x100
nvme_irq+0xfc/0x240 [nvme]
__handle_irq_event_percpu+0x40/0x180
handle_irq_event_percpu+0x30/0x80
handle_irq_event+0x36/0x53
handle_edge_irq+0x82/0x190
handle_irq+0xbf/0x100
do_IRQ+0x49/0xe0
common_interrupt+0xf/0xf