[CIFS] SessionSetup cleanup part 2

The cifs session setup code has three cases, and a fourth for backlevel
LANMAN2 style session setup needed to be added.  This new session setup
implmentation will eventually replace the other three and should be
easier to read while fixing a few minor problems (not setting
the LARGE READ/WRITEX flags when NTLMSSP was negotiated for example) and
adding support for NTLMv2 (which will be added with the next patch. In the
meantime, this code is marked in an CONFIG_CIFS_EXPERIMENTAL block and will
not be turned on by default until it is tested against more server types.

Signed-off-by: Steve French <sfrench@us.ibm.com>