Retire z_nr_znodes

Added in ab26409db753 ("Linux 3.1 compat, super_block->s_shrink"), with
the only consumer which needed the count getting retired in 066e82522101
("Linux compat: Minimum kernel version 3.10").

The counter gets in the way of not maintaining the list to begin with.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #15274