devfs: make destroy_dev() a release barrier for cdevpriv destructors runs

[kib]

Summary

Ensure that all destructors for cdevpriv finished running after
destroy_dev() returns to the caller.  Otherwise, since
devfs_destroy_cdevpriv() removes the cdevpriv data from the list, drops
the cdevpriv_mtx, and then starts the destructor, it is possible for
destroy_dev() to return before destructor finished in other thread.

This should allow drivers to safely remove cdev instance data that might
be referenced by cdevpriv data.

Diagnosed by:   kevans

[markj]

Ensure that all destructors for cdevpriv finished running after destroy_dev() returns to the caller.

s/after/before/

[kevans]

Thanks!