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.