LU-1934 ofd: implement precreate batching

LU-1934 ofd: implement precreate batching

Bulk precreate objects in a single tx handle.  For zfs this minimizes
the chance that the creates will be spread over multiple txgs.  This
can occur when the osts are under a concurrent write workload which
can consume bulk of the available space in a txg.

For ldiskfs this change shouldn't be harmful and the default value
can be set to 1 for ldiskfs if needed to get the previous behavior.

Change-Id: I81e310a8b630a1a29f3e017f6980901d6b6436fa
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4147
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>