l2arc_evict() is strictly serialized with respect to l2arc_write_buffers() and
l2arc_write_done().
Normally, l2arc_evict() and l2arc_write_buffers() are called from the same
thread, so they can not be concurrent.
Also, l2arc_write_buffers() uses zio_wait() on the parent zio of all cache zio-
s.
That ensures that l2arc_write_done() is completed before l2arc_write_buffers()
returns.
Finally, if a cache device is removed, then l2arc_evict() is called under
SCL_ALL in the exclusive mode.
That ensures that it can not be concurrent with the normal L2ARC accesses to
the device (including writing and evicting buffers).
Given the above, some checks and actions in l2arc_evict() do not make sense.
For instance, it must never encounter the write head header let alone remove it
from the buffer list.