scsi: ufs: Fix a deadlock in the error handler

In the Linux kernel, the following vulnerability has been resolved:

scsi: ufs: Fix a deadlock in the error handler

The following deadlock has been observed on a test setup:

- All tags allocated

- The SCSI error handler calls ufshcd_eh_host_reset_handler()

- ufshcd_eh_host_reset_handler() queues work that calls
ufshcd_err_handler()

- ufshcd_err_handler() locks up as follows:

Workqueue: ufs_eh_wq_0 ufshcd_err_handler.cfi_jt
Call trace:
__switch_to+0x298/0x5d8
__schedule+0x6cc/0xa94
schedule+0x12c/0x298
blk_mq_get_tag+0x210/0x480
__blk_mq_alloc_request+0x1c8/0x284
blk_get_request+0x74/0x134
ufshcd_exec_dev_cmd+0x68/0x640
ufshcd_verify_dev_init+0x68/0x35c
ufshcd_probe_hba+0x12c/0x1cb8
ufshcd_host_reset_and_restore+0x88/0x254
ufshcd_reset_and_restore+0xd0/0x354
ufshcd_err_handler+0x408/0xc58
process_one_work+0x24c/0x66c
worker_thread+0x3e8/0xa4c
kthread+0x150/0x1b4
ret_from_fork+0x10/0x30

Fix this lockup by making ufshcd_exec_dev_cmd() allocate a reserved
request.