NFS: Fix error handling for O_DIRECT write scheduling

If we fail to schedule a request for transmission, there are 2
possibilities:
1) Either we hit a fatal error, and we just want to drop the remaining
   requests on the floor.
2) We were asked to try again, in which case we should allow the
   outstanding RPC calls to complete, so that we can recoalesce requests
   and try again.

Fixes: d600ad1f ("NFS41: pop some layoutget errors to application")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>