CIFS: Introduce SMB2 Kconfig option

SMB2 is the followon to the CIFS (and SMB) protocols
and the default for Windows since Windows Vista, and also
now implemented by various non-Windows servers. SMB2
is more secure, has various performance advantages, including
larger i/o sizes, flow control, better caching model and more.
SMB2 also resolves some scalability limits in the CIFS
protocol and adds many new features while being much
simpler (only a few dozen commands instead of hundreds)
and since the protocol is clearer it is also more consistently
implemented across servers and thus easier to optimize.

After much discussion with Jeff Layton, Jeremy Allison
and others at Connectathon, we decided to move the SMB2
code from a distinct .ko and fstype into distinct
C files that optionally build in cifs.ko. As a result
the Kconfig gets simpler.

To avoid destabilizing CIFS, the SMB2 code is going
to be moved into its own experimental CONFIG_CIFS_SMB2 ifdef
as it is merged and rereviewed. The changes to stable
CIFS (builds with the SMB2 ifdef off) are expected to be
fairly small.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>