s3: smbd: Inside filename_convert_dirfsp_nosymlink(), don't require UCF_PREP_CREATEFI...

s3: smbd: Inside filename_convert_dirfsp_nosymlink(), don't require UCF_PREP_CREATEFILE when parsing a stream name that doesn't already exist.

We don't require it for a new file. Without this change, we have
to add UCF_PREP_CREATEFILE to the destination flags when we are
doing renames to a destination stream name, but not when doing
renames to a destination file name, which makes for inconsistent API use.

filename_convert_dirfsp() is now a drop in replacement
for filename_convert(), even for the ugly SMB1 POSIX
cases.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>