smbd: check for streams support in unix_convert()

smbd: check for streams support in unix_convert()

Fixes a regression introduced by the fixes for bug 15126 where we crash in
vfs_default in vfswrap_stat():

  assert failed: !is_named_stream(smb_fname)

The frontend calls into the VFS from build_stream_path() with a stream path
without checking if the share supports streams.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15126
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15161

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
[slow@samba.org: change from master adapted for unix_convert()]

Autobuild-User(v4-15-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-15-test): Tue Sep  6 07:31:51 UTC 2022 on sn-devel-184