vfs_fruit: Don't unlink the main file

vfs_fruit: Don't unlink the main file

The original fix for bug 13441 was missing a check that verifies that
fruit_ftruncate() is actually called on a stream.

Follow-up to

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13441

Pair-Programmed-With: Volker Lendecke <vl@samba.org>

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Thu Aug 23 15:28:48 CEST 2018 on sn-devel-144

(cherry picked from commit 8c14234871820eacde46670d722a676fb5f3a46c)