dm table: fix potential NULL pointer in dm_table_set_integrity
Commit a63a5cf (dm: improve block integrity support) introduced a
two-phase initialization of a DM device's integrity profile. The
dm_table_set_integrity() call to blk_integrity_register() may deference
a NULL 'template_disk' due to an integrity profile mismatch.
An integrity profile mismatch can occur if the integrity profile for
devices in a DM table are changed between the call to
dm_table_prealloc_integrity() and dm_table_set_integrity() (possible
via table loads for stacked DM devices with integrity profiles).