staging/lustre: Declare lu_context/session_tags_default

Make the declaration in a header, not as an extern in a C file,
that is frowned upon.
This also makes sparse a little bit more happy.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>