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/zfs/zfs_vnops.c

@@ -36,6 +36,7 @@
 #include <sys/sysmacros.h>
 #include <sys/resource.h>
 #include <sys/vfs.h>
+#include <sys/endian.h>
 #include <sys/vm.h>
 #include <sys/vnode.h>
 #include <sys/dirent.h>