vs_alloc can underflow in L2ARC vdevs

The current L2 ARC device code consistently uses psize to
increment vs_alloc but varies between psize and lsize when
decrementing it. The result of this behavior is that
vs_alloc can be decremented more that it is incremented
and underflow. This patch changes the code so asize is
used anywhere.

In addition, it ensures that vs_alloc gets incremented by
the L2 ARC device code as buffers are written and not at
the end of the l2arc_write_buffers() routine. The latter
(and old) way would temporarily underflow vs_alloc as
buffers that were just written, would be destroyed while
l2arc_write_buffers() was still looping.

Reviewed-by: Matt Ahrens <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Serapheim Dimitropoulos <[email protected]>
Closes #8298 

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