Rename refcount.h to zfs_refcount.h

Renamed to avoid conflicting with refcount.h when a different
implementation is already provided by the platform.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10620

module/zfs/vdev_initialize.c

@@ -27,7 +27,6 @@
 #include <sys/spa_impl.h>
 #include <sys/txg.h>
 #include <sys/vdev_impl.h>
-#include <sys/refcount.h>
 #include <sys/metaslab_impl.h>
 #include <sys/dsl_synctask.h>
 #include <sys/zap.h>