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: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>