libzpool: Disable -Wuse-after-free for dbuf.c.

[jhb]

Summary

The debug traces for reference counting in ZFS use the pointer of the
owning object as a "tag" for references to check that when an object
drops a reference it had actually held one. In a couple of places ZFS
drops references after freeing the owning object. In userland GCC
realizes this is a use after free. However, since only the value of
the pointer is used and it isn't indirected the use is harmless.

[emaste]

Maybe worth a brief comment beside CFLAGS.dbuf.c also, since based on the name this option seems like it would have no valid use.