pNFS/flexfiles: Fix layoutstat periodic reporting

Putting the periodicity timer in the mirror instances is causing
non-scalable reporting behaviour and missed reporting intervals.
When you recall layouts and/or implement client side mirroring, it
leads to consecutive reports with only a few ms between RPC calls.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Fixes: d0379a5d ("pNFS/flexfiles: Support server-supplied...")