staging: lustre: Fix do not use C99 // comments in mgc_request.c

This patch fixed "ERROR: do not use C99 // comments"
found by checkpatch.pl in lustre/lustre/mgc_request.c

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>