Instead of user_path_walk() and comparing dentries, sys_swapoff()
opens its argument and compares ->i_mapping.  Result: slightly
simpler code and swapoff(2) becomes tolerant to e.g.

	swapon /dev/sda2
	switch root from initrd to sda1
	....
	swapoff /dev/sda2  # where /dev is from sda1, not from initrd

current tree fails in the case above (different dentries -> no love).