All filesystems were converted to dynamically allocated BDIs. The
destruction of backing_dev_info structures is handled as part of
super block destruction. Refactor the code to abstract away the
details of creating and destroying a BDI.
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#6089
7dae2c81e7