Improve spinning in mtx_enter().

Improve spinning in mtx_enter().

Instead of calling mtx_enter_try() in each spinning loop, do it
only if the result of a lockless read indicates that the mutex has
been released.  This avoids some expensive atomic compare-and-swap
operations.  Up to 5% reduction of spinning time during kernel build
can been seen on a 8 core amd64 machine.  On other machines there
was no visible effect.

Test on powerpc64 has revealed a bug in mtx_owner declaration.  Not
the variable was volatile, but the object it points to.  Move the
volatile declaration in struct mutex to avoid a hang when going to
multiuser.

from Mateusz Guzik; input kettenis@ jca@; OK mpi@