s3:smbd: add SMB_VFS_READDIR_ATTR() to marshall direntry

s3:smbd: add SMB_VFS_READDIR_ATTR() to marshall direntry

SMB_VFS_READDIR_ATTR is a last minute hook to fetch additional metadata
for a directory entry when we're already marshalling the SMB reply buffer.

This would we used, when there's a need to repurpose some fields in the
the reply, like it's done with Apple's SMB2 extension "AAPL".

We then fetch AAPL metadata with the shiny new SMB_VFS_READDIR_ATTR()
VFS call and marshall appropiately.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>