Change l2arc_norw default to zero

These days modern SSDs can efficiently service concurrent reads
and writes. When this flag was added that wasn't really the
case for a variety of SSD controllers. But now we can set the
default value to take advantage of this parallelism and only
disable this as needed for specific troublesome hardware.

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