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.
v4: Fix alignment of log block.

Close #925

Ported-by: Yuxuan Shui <yshuiv7@gmail.com>