cifs: Retain old ACEs when converting between mode bits and ACL.

When cifsacl mount option is used, retain the ACEs which
should not be modified during chmod. Following is the approach taken:

1. Retain all explicit (non-inherited) ACEs, unless the SID is one
of owner/group/everyone/authenticated-users. We're going to set new
ACEs for these SIDs anyways.
2. At the end of the list of explicit ACEs, place the new list of
ACEs obtained by necessary conversion/encoding.
3. Once the converted/encoded ACEs are set, copy all the remaining
ACEs (inherited) into the new ACL.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>