NFSv4: Fix a sleep in atomic context in nfs4_callback_sequence()

NFSv4: Fix a sleep in atomic context in nfs4_callback_sequence()

commit 8618289c46556fd4dd259a1af02ccc448032f48d upstream.

We must drop the lock before we can sleep in referring_call_exists().

Reported-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Fixes: 045d2a6d076a ("NFSv4.1: Delay callback processing...")
Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>