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/os/linux/zfs/arc_os.c

@@ -33,7 +33,7 @@
 #include <sys/zio_checksum.h>
 #include <sys/zfs_context.h>
 #include <sys/arc.h>
-#include <sys/refcount.h>
+#include <sys/zfs_refcount.h>
 #include <sys/vdev.h>
 #include <sys/vdev_trim.h>
 #include <sys/vdev_impl.h>

module/os/linux/zfs/zfs_znode.c

@@ -54,7 +54,7 @@
 #include <sys/dmu.h>
 #include <sys/dmu_objset.h>
 #include <sys/dmu_tx.h>
-#include <sys/refcount.h>
+#include <sys/zfs_refcount.h>
 #include <sys/stat.h>
 #include <sys/zap.h>
 #include <sys/zfs_znode.h>