cifs: Fix chmod with modefromsid when an older ACE already exists.

My recent fixes to cifsacl to maintain inherited ACEs had
regressed modefromsid when an older ACL already exists.

Found testing xfstest 495 with modefromsid mount option

Fixes: f5065508

 ("cifs: Retain old ACEs when converting between mode bits and ACL")

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>