SMB3 mounts fail with access denied to some servers

We were checking incorrectly if signatures were required to be sent,
so were always sending signatures after the initial session establishment.
For SMB3 mounts (vers=3.0) this was a problem because we were putting
SMB2 signatures in SMB3 requests which would cause access denied
on mount (the tree connection would fail).

This might also be worth considering for stable (for 3.7), as the
error message on mount (access denied) is confusing to users and
there is no workaround if the server is configured to only
support smb3.0. I am ok either way.

CC: stable <stable@kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>