ksmbd: fix maximum allowed access checks

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: fix maximum allowed access checks

The DACL permission check looks for an ACE matching the current user and
falls back to the Everyone ACE. It does not consider an Authenticated
Users ACE, even though an authenticated session is a member of that
well-known group.

As a result, opening a file whose access is granted through S-1-5-11 can
incorrectly fail with STATUS_ACCESS_DENIED. Treat an Authenticated Users
ACE as a fallback entry alongside Everyone.

The maximal access calculation also combines access masks from every ACE,
regardless of whether its SID applies to the current user. This can grant
rights belonging to an unrelated principal. Process only ACEs applying to
the user, Everyone, or Authenticated Users, and accumulate allowed and
denied masks in ACL order. Preserve explicitly requested access bits so
they are validated against the resulting maximal mask.

When ACCESS_SYSTEM_SECURITY is denied, report STATUS_PRIVILEGE_NOT_HELD
instead of the generic STATUS_ACCESS_DENIED. Access to the system ACL
requires a security privilege that ksmbd does not grant.

For regular files, include FILE_EXECUTE in maximal access when the client
requested GENERIC_EXECUTE and the DACL grants the complete file-read set.
Keep a direct FILE_EXECUTE request subject to the explicit DACL bit. This
matches the POSIX file ACL mapping without broadening specific execute
requests.

Do not replace rights from an applicable NT ACE with a POSIX ACL entry.
The POSIX ACL is only a fallback when no user, Everyone, or Authenticated
Users ACE applies; otherwise it can incorrectly broaden the stored DACL.

This fixes smb2.maximum_allowed.maximum_allowed.