Fix a bug introduced in
https://svnweb.freebsd.org/base?view=revision&revision=272347

MFC after: 3 days
revert r272702: wrong (earlier) change was committed
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.

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