pNFS/flexfiles: don't attempt pnfs on fatal DS errors

pNFS/flexfiles: don't attempt pnfs on fatal DS errors

When an applications get killed (SIGTERM/SIGINT) while pNFS client performs a connection
to DS, client ends in an infinite loop of connect-disconnect. This
source of the issue, it that flexfilelayoutdev#nfs4_ff_layout_prepare_ds gets an error
on nfs4_pnfs_ds_connect with status ERESTARTSYS, which is set by rpc_signal_task, but
the error is treated as transient, thus retried.

The issue is reproducible with Ctrl+C the following script(there should be ~1000 files in
a directory, client should must not have any connections to DSes):

```
echo 3 > /proc/sys/vm/drop_caches

for i in *
do
   head -1 $i
done
```

The change aims to propagate the nfs4_ff_layout_prepare_ds error state
to the caller that can decide whatever this is a retryable error or not.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Link: https://lore.kernel.org/r/20250627071751.189663-1-tigran.mkrtchyan@desy.de
Fixes: 260f32adb88d ("pNFS/flexfiles: Check the result of nfs4_pnfs_ds_connect")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>