3525 persistent l2arc.

For details, see: https://www.illumos.org/issues/3525

v2: Change two KM_SLEEP to KM_PUSHPAGE.
v3: Change one more KM_SLEEP.

Close #925

Ported-by: Yuxuan Shui <yshuiv7@gmail.com>
Merge branch 'illumos-3525' of https://github.com/yshui/zfs into perf

Conflicts:
	module/zfs/arc.c