uml: Create a private mount of proc for mconsole

The mconsole code only ever accesses proc for the initial pid
namespace.  Instead of depending upon the proc_mnt which is
for proc_flush_task have uml create it's own mount of proc
instead.

This allows proc_flush_task to evolve and remove the
need for having a proc_mnt to do it's job.

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>