scsi: ufs: core: Remove SCSI host only if added

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

scsi: ufs: core: Remove SCSI host only if added

If host tries to remove ufshcd driver from a UFS device it would cause a

kernel panic if ufshcd_async_scan fails during ufshcd_probe_hba before

adding a SCSI host with scsi_add_host and MCQ is enabled since SCSI host

has been defered after MCQ configuration introduced by commit 0cab4023ec7b

("scsi: ufs: core: Defer adding host to SCSI if MCQ is supported").

To guarantee that SCSI host is removed only if it has been added, set the

scsi_host_added flag to true after adding a SCSI host and check whether it

is set or not before removing it.