vfs_fruit: move check in ad_convert() to ad_convert_*() subfunctions

vfs_fruit: move check in ad_convert() to ad_convert_*() subfunctions

Currently the whole conversion is skipped if the FinderInfo entry in the
AppleDouble file is of the default size (ie not containing xattrs).

That also means we never converted FinderInfo from the AppleDouble file
to stream format. This change finally fixes this.

Note that this keeps failing with streams_depot, much like the existing
known-fail of "samba3.vfs.fruit streams_depot.OS X AppleDouble file
conversion". Fixing the conversion to work with vfs_streams_depot is a
task for another day.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13649

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Thu Oct 11 01:30:13 CEST 2018 on sn-devel-144