vfs_streams_xattr: fix and simplify streams_xattr_get_name()

vfs_streams_xattr: fix and simplify streams_xattr_get_name()

streams_xattr_get_name() fails to chop off the stream type in case
config->store_stream_type is false and the passed stream name contains a
stream type.

Eg when the passed in stream name is ":mystream:$DATA", but
config->store_stream_type is false, we must generate a xattr name of
"mystream" or "user.mystream".

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11466

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Oct 16 23:27:01 CEST 2015 on sn-devel-104