cifs: prevent NULL deref in cifs_compose_mount_options()

cifs: prevent NULL deref in cifs_compose_mount_options()

BugLink: https://bugs.launchpad.net/bugs/1940139
[ Upstream commit 03313d1c3a2f086bb60920607ab79ac8f8578306 ]

The optional @ref parameter might contain an NULL node_name, so
prevent dereferencing it in cifs_compose_mount_options().

Addresses-Coverity: 1476408 ("Explicit null dereferenced")
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>