pNFS/flexfiles: Ensure we record layoutstats even if RPC is terminated early

Currently, we will only record the layoutstats correctly if the
RPC call successfully obtains a slot. If we exit before that
happens, then we may find ourselves starting the busy timer through
the call in ff_layout_(read|write)_prepare_layoutstats, but never stopping it.

The same thing happens if we're doing DA-DS.

The fix is to ensure that we catch these cases in the rpc_release()
callback.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>