NFS: Do not report EINTR/ERESTARTSYS as mapping errors

If the attempt to flush data was interrupted due to a local signal, then
just requeue the writes back for I/O.

Fixes: 6fbda89b ("NFS: Replace custom error reporting mechanism with generic one")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>