Add zfs_mdcomp_disable module option

Expose the zfs_mdcomp_disable variable as a module option. This
can be used to disable compression of zfs meta data which is
enabled by default. This shouldn't need to be tuned but for
most workloads, however there may be very specific instances
where it makes sense to trade disk capacity for extra cpu cycles.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>