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The reference counting of dax_region objects is needlessly complicated,
has lead to confusion [1], and has hidden a bug [2]. Towards cleaning up
that mess introduce alloc_dev_dax_id() to minimize the holding of a
dax_region reference to only what dev_dax_release() needs, the
dax_region->ida.

Part of the reason for the mess was the design to dereference a
dax_region in all cases in free_dev_dax_id() even if the id was
statically assigned by the upper level dax_region driver. Remove the
need to call "is_static(dax_region)" by tracking whether the id is
dynamic directly in the dev_dax instance itself.

With that flag the dax_region pinning and release per dev_dax instance
can move to alloc_dev_dax_id() and free_dev_dax_id() respectively.

A follow-on cleanup address the unnecessary references in the dax_region
setup and drivers.

Fixes: 0f3da14a ("device-dax: introduce 'seed' devices")
Link: http://lore.kernel.org/r/20221203095858.612027-1-liuyongqiang13@huawei.com [1]
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