pnfs/flexfiles: Fix PTR_ERR() dereferences in ff_layout_track_ds_error

mirror->mirror_ds can be NULL if uninitialised, but can contain
a PTR_ERR() if call to GETDEVICEINFO failed.

Fixes: 65990d1a ("pNFS/flexfiles: Fix a deadlock on LAYOUTGET")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org # 4.10+