staging/lustre/ptlrpc: Move nrs_conf_fifo extern to a header

This avoids having an extern definition in a C file which is bad,
and also silences sparse complaint as well.

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