orangefs: use list_for_each_entry_safe in purge_waiting_ops

commit 0afc0decf247f65b7aba666a76a0a68adf4bc435 upstream.

set_op_state_purged can delete the op.

Signed-off-by: Martin Brandenburg <martin@omnibond.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>