Fix unused variable warning when CIFS POSIX disabled

Fix unused variable warning when CONFIG_CIFS_POSIX disabled.

   fs/cifs/ioctl.c: In function 'cifs_ioctl':
>> fs/cifs/ioctl.c:40:8: warning: unused variable 'ExtAttrMask' [-Wunused-variable]
     __u64 ExtAttrMask = 0;
           ^
Pointed out by 0-DAY kernel build testing backend

Signed-off-by: Steve French <smfrench@gmail.com>