There are disk devices that have logical sector size larger than 512B, for
example 4KB. That is, their physical sector size is larger than 512B and they
do not provide emulation for 512B sector sizes. For such devices both a data
offset and a data size must be properly aligned. L2ARC should arrange that
because it uses physical I/O.
zio_vdev_io_start() performs a necessary transformation if io_size is not
aligned to vdev_ashift, but that is done only for logical I/O. Something
similar should be done in L2ARC code.
    * a temporary write buffer should be allocated if the original buffer is
      not going to be compressed and its size is not aligned
    * size of a temporary compression buffer should be ashift aligned
    * for the reads, if a size of a target buffer is not sufficiently large and
      it is not aligned then a temporary read buffer should be allocated