l2arc: restore correct rounding up of asize of compressed data

This rounding up was lost in a mismerge of illumos code.
See r268075 MFV r267565.
After that commit zio_compress_data() no longer performs any compressed
size adjustment, so it needs to be done externally.  On FreeBSD we round
up the size using vdev_ashift rather than SPA_MINBLOCKSIZE so that 4KB
devices are properly supported.

Additionally, zero out the buffer tail only if compression succeeds.
The compression is considered successful if the size of compressed
data after rounding up to account for the vdev ashift is less than the
original data size.  It does not make sense to have the data compressed
if all the savings are lost to rounding up.
With the new zio_compress_data() it could have been possible that the
rounded compressed size would be greater than the original size and thus
we could zero beyond the allocated buffer if the zeroing code was kept
at the original place.

Discussed with:	delphij, gibbs
MFC after:	2 weeks
X-MFC with:	r274627
Re-introduce padding fields removed with r264321 to keep
struct tcptw ABI unchanged.

Suggested by:	jhb
Approved by:	jhb (mentor)
MFC after:	1 day
X-MFC-With:	r264321