s3:smbd: work around broken "vfs mkdir use tmp name" on FAT

s3:smbd: work around broken "vfs mkdir use tmp name" on FAT

"vfs mkdir use tmp name" creates a name with ":" because the file should
be invisible for Windows clients. ":" however is an invalid character on
FAT filesystems and we get EINVAL back. In that case we fall back to not
using tmp names for mkdir.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15845

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Bjoern Jacke <bjacke@samba.org>