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/freebsd/spl/spl_vfs.c

@@ -40,6 +40,7 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/mutex.h>
 #include <sys/vnode.h>
+#include <sys/taskq.h>
 
 #include <sys/ccompat.h>