Revert "reduce L2ARC_WRITE_SIZE on FreeBSD"

This reverts commit 441e753ad7a53cf5218cdc89ae4b85f25449d778.

Changing L2ARC_WRITE_SIZE affects how much data is actually written
to l2 cache during each pass, but the intention was to change only
how much data is scanned.
l2arc_write_buffers: reduce headroom value

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.

headroom determines how much data is scanned on a single list
during each run of the l2arc feed thread.
Because FreeBSD has more lists we proportionally decrease the limit.