s3: smbd - Fix SMB3.11 protocol encryption selection.

s3: smbd - Fix SMB3.11 protocol encryption selection.

Selecting encryption in 3.11 depends on the negprot contexts being present.
Setting SMB2_CAP_ENCRYPTION from the 3.11 client is optional. The absence
of it should not remove the negprot context.

Found by the Microsoft testsuites at the Redmond plugfest.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Steve French <sfrench@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Jun 18 04:09:04 CEST 2015 on sn-devel-104