cifs: fix potential use-after-free bugs in TCP_Server_Info::hostname

cifs: fix potential use-after-free bugs in TCP_Server_Info::hostname

BugLink: https://bugs.launchpad.net/bugs/2025067
commit 90c49fce1c43e1cc152695e20363ff5087897c09 upstream.

TCP_Server_Info::hostname may be updated once or many times during
reconnect, so protect its access outside reconnect path as well and
then prevent any potential use-after-free bugs.

Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>