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

include/sys/spa_impl.h

@@ -44,7 +44,7 @@
 #include <sys/uberblock_impl.h>
 #include <sys/zfs_context.h>
 #include <sys/avl.h>
-#include <sys/refcount.h>
+#include <sys/zfs_refcount.h>
 #include <sys/bplist.h>
 #include <sys/bpobj.h>
 #include <sys/dsl_crypt.h>