mount.zfs is called by convention (and util-linux) with arguments last, i.e.
  % mount.zfs <dataset> <mountpoint> -o <options>
Use getopt_long so musl will permute arguments.

Passing 0 as long_opts is unspecified, but safe for glibc and musl getopt_long.

--- a/cmd/mount_zfs/mount_zfs.c	2016-01-14 13:55:43.352951688 +0100
+++ b/cmd/mount_zfs/mount_zfs.c	2016-01-14 13:53:35.685221569 +0100
@@ -33,6 +33,7 @@
 #include <libzfs.h>
 #include <locale.h>
 #include <fcntl.h>
+#include <getopt.h>
 
 #define	ZS_COMMENT	0x00000000	/* comment */
 #define	ZS_ZFSUTIL	0x00000001	/* caller is zfs(8) */
@@ -390,7 +391,7 @@
 	opterr = 0;
 
 	/* check options */
-	while ((c = getopt(argc, argv, "sfnvo:h?")) != -1) {
+	while ((c = getopt_long(argc, argv, "sfnvo:h?", 0, 0)) != -1) {
 		switch (c) {
 		case 's':
 			sloppy = 1;