pNFS: Add a flag argument to pnfs_destroy_layouts_byclid()

Change the bool argument to a flag so that we can add different modes
for doing bulk destroy of a layout. In particular, we will want the
ability to schedule return of all the layouts associated with a given
NFS server when it reboots.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>