staging/lustre: Move lov_read_and_clear_async_rc declaration

Move it to obd.h, so that it's included from both the users and
the actual definition, making sure they never get out of sync.
This also silences a sparse warning.

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