scst_vdisk: Refresh thin provisioning after backend changes

[lnocturno]

Changing filename while a vdisk is inactive leaves the old backend's thin-provisioning capabilities cached. For example, switching between a plain device and a discard-capable cache can leave automatic thin provisioning disabled or enabled for the wrong backend.

Mark only a TP recheck as pending after a successful filename write. Consume it in the common activation path used by active=1 and ALUA. Do not request a full backend reexamination for an existing device: that would also overwrite the exported size and recheck cache mode. The initial deferred reexamination still performs its normal work; if it completes the TP check, activation does not repeat that check.

Make the TP probe safe to call repeatedly:

• Track successful creation of gen_tp_soft_threshold_reached_UA with tp_ua_attr_created. This avoids duplicate sysfs creation for both thin-to-thin switches and thin-to-non-thin-to-thin round trips. Keep the attribute until device removal; its existing store callback rejects requests while TP is disabled. Failed creation leaves the flag clear so a later probe can retry.
• Compare the previous effective TP state and advertised UNMAP limits with vdisk_tp_changed(). Notify on TP enable/disable and on changed granularity, alignment, maximum LBA count or discard-zeroes behavior while TP remains enabled. Ignore stale limits while TP is disabled.
• Preserve the existing manual-setting policy, including an explicit thin_provisioned=0. A backend swap does not unconditionally enable TP.
• On a probe-open error, leave the previous capabilities and pending flag intact. Activation closes any reopened descriptors and returns the device to inactive state so a subsequent activation can retry.

Filename replacement, explicit activation and ALUA activation already serialize through scst_alua_lock(). Use that same lock for the new state so another filename write cannot race with consuming the pending check. Keep the new bool fields outside the command-side bitfields to avoid sharing their read-modify-write storage with command callbacks.

Queue INQUIRY DATA HAS CHANGED through vdev_inq_changed_work. Calling scst_dev_inquiry_data_changed() directly from activation could recurse on scst_mutex. The existing device-free callback cancels this work before releasing the device. Ordinary ALUA or active cycles without a filename write do not request another TP probe or notification.

Link: #389