cifs: silence printk when establishing first session on socket

When signing is enabled, the first session that's established on a
socket will cause a printk like this to pop:

    CIFS VFS: Unexpected SMB signature

This is because the key exchange hasn't happened yet, so the signature
field is bogus. Don't try to check the signature on the socket until the
first session has been established. Also, eliminate the specific check
for SMB_COM_NEGOTIATE since this check covers that case too.

Cc: stable@kernel.org
Cc: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>