MFV r269224:

Increase default ARC buf_hash_table size.  When typical block size is small,
the hash table could be too small, which would lead to long hash chains and
limit performance for cached reads.

A new loader tunable, vfs.zfs.arc_average_blocksize, have been added which
allows users to override the default assumption of average (typical) block
size.  Old default was 65536 (64 KiB) and new default is 8192 (8 KiB).

Illumos issue:
    5034 ARC's buf_hash_table is too small

MFC after:	2 weeks