smb: client: fix warning in cifs_smb3_do_mount()

This fixes the following warning reported by kernel test robot

  fs/smb/client/cifsfs.c:982 cifs_smb3_do_mount() warn: possible
  memory leak of 'cifs_sb'

Link: https://lore.kernel.org/all/202306170124.CtQqzf0I-lkp@intel.com/


Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>