fix smb3-encryption breakage when CONFIG_DEBUG_SG=y

We can not use the standard sg_set_buf() fucntion since when
CONFIG_DEBUG_SG=y this adds a check that will BUG_ON for cifs.ko
when we pass it an object from the stack.

Create a new wrapper smb2_sg_set_buf() which avoids doing that particular check
and use it for smb3 encryption instead.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
CC: Stable <stable@vger.kernel.org>