MFC r256889:

Use the vdev's ashift to calculate the supported min block size passed to
zio_compress_data(..) when compressing l2arc buffers.

This eliminates L2ARC I/O errors, which resulted in very poor performance on
vdev's configured with block size greater than 512b due to compression
assuming a smaller min block size than the vdev supports.

Approved by: re (glebius)