FBSD: use better casts in vop_advlock

FBSD: use better casts in vop_advlock

In the bsd.kmod.mk world, the compiler bails when we cast a pointer
directly to int.  Cast through intptr_t as the supported mechanism.
On amd64, this loses bits, but since this instance is just attempting
to use the value as a unique handle, it is probably okay for now.
However, it should be addressed more properly eventually, when
this locking implementation sees wider use.

Change-Id: I4fe8084c14a97dc4efc8d74e9971b1540c028e40
Reviewed-on: http://gerrit.openafs.org/5034
Reviewed-by: Matt Benjamin <matt@linuxbox.com>
Reviewed-by: Garrett Wollman <wollman@csail.mit.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>