cifs: modefromsid: make room for 4 ACE

when mounting with modefromsid, we end up writing 4 ACE in a security
descriptor that only has room for 3, thus triggering an out-of-bounds
write. fix this by changing the min size of a security descriptor.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>