cifs: remove set but not used variable 'cifs_sb'

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

fs/cifs/ioctl.c: In function 'cifs_ioctl':
fs/cifs/ioctl.c:164:23: warning:
 variable 'cifs_sb' set but not used [-Wunused-but-set-variable]

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