functionfs: need to cancel ->reset_work in ->kill_sb()

... otherwise we just might free ffs with ffs->reset_work
still on queue.  That needs to be done after ffs_data_reset() -
that's the cutoff point for configfs accesses (serialized
on gadget_info->lock), which is where the schedule_work()
would come from.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>