uml: LDT mutex conversion

The ldt.semaphore conforms to the new struct mutex requirments, so I converted
it to use the new API and changed the name.

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>