LU-4865 zfs: grow block size by write pattern

This patch grows the block size by write RPC. The osd-zfs blocksize
used to be fixed at 128KB, which is too big for random write and
too small for seqential write.

This patch decides the block size by the first few RPCs. If the first
few RPCs are sequential, mostly it will pick maximum block size for
the object; otherwise, a feasible block size will be picked by the
RPC size.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I66f7cbdc2b5e0365058b152b4865b00cdabb0cf3
Reviewed-on: http://review.whamcloud.com/18441
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Don Brady <don.brady@intel.com>