Initialize it during command allocation.

Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Stephen Bates <sbates@raithlin.com>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>