Linux 5.20 compat: bdevname()

As of the Linux 5.20 kernel bdevname() has been removed, all
callers should use snprintf() and the "%pg" format specifier.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13728