uml: Don't consult current to find the proc_mnt in mconsole_proc

Inspection of the control flow reveals that mconsole_proc is either
called from mconsole_stop called from mc_work_proc or from
mc_work_proc directly.  The function mc_work_proc is dispatched to a
kernel thread with schedule_work.

All of the threads that run dispatched by schedule_work are in the
init pid namespace.

So make the code clearer and by using init_pid_ns instead of
task_active_pid_ns(current).

Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>