Use setmntent() OR fopen()

For the same reasons it's used in libzfs_init(), this was just
overlooked because zinject gets minimal use.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs#1498

cmd/zinject/translate.c

@@ -115,7 +115,11 @@ parse_pathname(const char *inpath, char *dataset, char *relpath,
 		return (-1);
 	}
 
+#ifdef HAVE_SETMNTENT
+	if ((fp = setmntent(MNTTAB, "r")) == NULL) {
+#else
 	if ((fp = fopen(MNTTAB, "r")) == NULL) {
+#endif
 		(void) fprintf(stderr, "cannot open /etc/mtab\n");
 		return (-1);
 	}