fs/nfs: Use fatal_signal_pending instead of signal_pending

We set the state of the current process to TASK_KILLABLE via
prepare_to_wait(). Should we use fatal_signal_pending() to detect
the signal here?

Fixes: b4868b44

 ("NFSv4: Wait for stateid updates after CLOSE/OPEN_DOWNGRADE")
Signed-off-by: zhouchuangao <zhouchuangao@vivo.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>