mtd: nand: make suspend work if device is accessed by kernel threads.

mtd: nand: make suspend work if device is accessed by kernel threads.

Since all userspace threads are frozen at the time the nand_suspend is called,
they aren't inside any nand function.

We don't call try_to_freeze in nand ether. Thus the only user that can
be inside the nand functions is an non freezeable kernel thread. Thus we
can safely wait for it to finish.

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>