uml: port mutex conversion

The port_sem is already used as a mutex since it's using DECLARE_MUTEX(), but
the underlying construct is still a semaphore ..  This patch switches it over
to a struct mutex.

Signed-off-by: Daniel Walker <dwalker@mvista.com>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>