cifs: remove set but not used variable 'smb_buf'

Fixes gcc '-Wunused-but-set-variable' warning:

fs/cifs/sess.c: In function '_sess_auth_rawntlmssp_assemble_req':
fs/cifs/sess.c:1157:18: warning:
 variable 'smb_buf' set but not used [-Wunused-but-set-variable]

It never used since commit cc87c47d ("cifs: Separate rawntlmssp auth
from CIFS_SessSetup()")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Steve French <stfrench@microsoft.com>