Make zil_slog_limit a tunable module parameter.

zil_slog_limit specifies the maximum commit size to be written to the separate
log device. Larger commits bypass the separate log device and go directly to
the data devices.

The optimal value for zil_slog_limit directly depends on the latency and
throughput characteristics of both the separate log device and the data disks.
Small synchronous writes are faster on low-latency separate log devices (e.g.
SSDs) whereas large synchronous writes are faster on high-latency data disks
(e.g. spindles) because of higher throughput, especially with a large array.
The point is, the line between "small" and "large" synchronous writes in this
scenario is heavily dependent on the hardware used. That's why it should be
made configurable.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #783