cifs: remove unneeded NULL checks from cifs_show_options

show_options is always called with the namespace_sem held. Therefore we
don't need to worry about the vfsmount being NULL, or it vanishing while
the function is running. By the same token, there's no need to worry
about the superblock, tcon, smb or tcp sessions being NULL on entry.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>