LU-10143 osd-zfs: allocate sequence in advance

LU-10143 osd-zfs: allocate sequence in advance

on the controller, so that we have it ready before any potential
read-only makeup. this is what osd-ldiskfs is doing already.

Change-Id: I3d27f112b0d013ac923c5d250b296b5528b8112d
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34069
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>