cifs: Fix NULL pointer dereference of devname

There is a NULL pointer dereference of devname in strspn()

The oops looks something like:

  CIFS: Attempting to mount (null)
  BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
  ...
  RIP: 0010:strspn+0x0/0x50
  ...
  Call Trace:
   ? cifs_parse_mount_options+0x222/0x1710 [cifs]
   ? cifs_get_volume_info+0x2f/0x80 [cifs]
   cifs_setup_volume_info+0x20/0x190 [cifs]
   cifs_get_volume_info+0x50/0x80 [cifs]
   cifs_smb3_do_mount+0x59/0x630 [cifs]
   ? ida_alloc_range+0x34b/0x3d0
   cifs_do_mount+0x11/0x20 [cifs]
   mount_fs+0x52/0x170
   vfs_kern_mount+0x6b/0x170
   do_mount+0x216/0xdc0
   ksys_mount+0x83/0xd0
   __x64_sys_mount+0x25/0x30
   do_syscall_64+0x65/0x220
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fix this by adding a NULL check on devname in cifs_parse_devname()
Signed-off-by: NYao Liu <yotta.liu@ucloud.cn>
Signed-off-by: NSteve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -1486,6 +1486,11 @@ cifs_parse_devname(const char *devname, struct smb_vol *vol)
 	const char *delims = "/\\";
 	size_t len;
 
+	if (unlikely(!devname || !*devname)) {
+		cifs_dbg(VFS, "Device name not specified.\n");
+		return -EINVAL;
+	}
+
 	/* make sure we have a valid UNC double delimiter prefix */
 	len = strspn(devname, delims);
 	if (len != 2)