Patch from: Eric Lammerts <eric@lammerts.org>

Every time you do a loop mount, a kernel thread is started (those
processes are called "loop0", "loop1", etc.). The problem is that when
it starts, it's counted as one of your processes. Then, it's
changed to be a root-owned process without correcting that count.

Patch below fixes the problem. It moves the bookkeeping of changing
current->user to a new function switch_uid() (which is now also used
by exec_usermodehelper() in kmod.c). The patch is tested.