orangefs: fix a oob in orangefs_debug_write

I got a syzbot report: slab-out-of-bounds Read in
orangefs_debug_write... several people suggested fixes,
I tested Al Viro's suggestion and made this patch.

Signed-off-by: Mike Marshall <hubcap@omnibond.com>
Reported-by:  <syzbot+fc519d7875f2d9186c1f@syzkaller.appspotmail.com>