cifs: fix rmmod regression in cifs.ko caused by force_sig changes

stable inclusion
from linux-4.19.99
commit 7f6a96dd8223796ffae4dd251be3bff161a28a4b
bugzilla: https://bugzilla.openeuler.org/show_bug.cgi?id=27
CVE: NA
---------------------------

[ Upstream commit 247bc947 ]

Fixes: 72abe3bc ("signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force_sig")

The global change from force_sig caused module unloading of cifs.ko
to fail (since the cifsd process could not be killed, "rmmod cifs"
now would always fail)
Signed-off-by: NSteve French <stfrench@microsoft.com>
Reviewed-by: NRonnie Sahlberg <lsahlber@redhat.com>
CC: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
[io_uring need allow_kernel_signal]
Signed-off-by: Nyangerkun <yangerkun@huawei.com>
Reviewed-by: Nzhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>

fs/cifs/connect.c

@@ -974,6 +974,7 @@ cifs_demultiplex_thread(void *p)
 		mempool_resize(cifs_req_poolp, length + cifs_min_rcv);
 
 	set_freezable();
+	allow_signal(SIGKILL);
 	while (server->tcpStatus != CifsExiting) {
 		if (try_to_freeze())
 			continue;