MFC r337007: MFV r336991, r337001:

MFC r337007: MFV r336991, r337001:
9102 zfs should be able to initialize storage devices

The first access to a disk block can incur a performance penalty on some
platforms (e.g. AWS's EBS, VMware VMDKs). Therefore it is recommended that
volumes be "thick provisioned", where supported by the platform (VMware).
Thick provisioning is time consuming and often is ignored. If the thick
provision step is omitted, customers will see suboptimal performance until
we have written to all parts of the LUN. ZFS should be able to initialize
any unused storage to remove any first-write penalty that exists.

illumos/illumos-gate@094e47e980b0796b94b1b8f51f462a64d246e516

Reviewed by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author:     George Wilson <george.wilson@delphix.com>