l2arc: make sure that all writes honor ashift of a cache device

Previously uncompressed buffers did not obey that rule.

Type of b_asize is changed to uint64_t for consistency,
given that this is a zeta-byte filesystem.

l2arc_compress_buf is renamed to l2arc_transform_buf to better reflect
its new utility. Now not only we ensure that a compressed buffer has
a size aligned to ashift, but we also allocate a properly sized
temporary buffer if the original buffer is not compressed and it has
an odd size. This ensures that all I/O to the cache device is always
ashift-aligned, in terms of both a request offset and a request size.

If the aligned data is larger than the original data, then we have to use
a temporary buffer when reading it as well.

Also, enhance physical zio alignment checks using vdev_logical_ashift.
On FreeBSD we have this information, so we can make stricter assertions.

Reviewed by: smh, mav
MFC after: 1 month
Sponsored by: ClusterHQ
Differential Revision: https://reviews.freebsd.org/D2789