cifs: fix GlobalMid_Lock bug in cifs_reconnect

We can not hold the GlobalMid_Lock spinlock during the
dfs processing in cifs_reconnect since it invokes things that may sleep
and thus trigger :

BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:23

Thus we need to drop the spinlock during this code block.

RHBZ: 1716743

Cc: stable@vger.kernel.org
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -476,6 +476,7 @@ cifs_reconnect(struct TCP_Server_Info *server)
 	spin_lock(&GlobalMid_Lock);
 	server->nr_targets = 1;
 #ifdef CONFIG_CIFS_DFS_UPCALL
+	spin_unlock(&GlobalMid_Lock);
 	cifs_sb = find_super_by_tcp(server);
 	if (IS_ERR(cifs_sb)) {
 		rc = PTR_ERR(cifs_sb);
@@ -493,6 +494,7 @@ cifs_reconnect(struct TCP_Server_Info *server)
 	}
 	cifs_dbg(FYI, "%s: will retry %d target(s)\n", __func__,
 		 server->nr_targets);
+	spin_lock(&GlobalMid_Lock);
 #endif
 	if (server->tcpStatus == CifsExiting) {
 		/* the demux thread will exit normally