reduce L2ARC_WRITE_SIZE on FreeBSD

FreeBSD has ARC_BUFC_NUMMETADATALISTS metadata lists and ARC_BUFC_NUMDATALISTS
data lists (currently both are 16) while illumos has just a single list
of each kind.

L2ARC_WRITE_SIZE determines the default value of l2arc_write_max which
defines limits on how much data is scanned and written to a cache device
during each run of the l2arc feed thread.  The limits are applied on the
per buffer list basis.
Because FreeBSD has more lists we proportionally reduce the limits.

Reviewed by:	Brendan Gregg (earlier version)
MFC after:	2 weeks
Sponsored by:	HybridCluster