libspl/mnttab: remove struct extmnttab

The two additional fields are never used by calling code, and we can
replace their sole internal use with an extra stack param.

Sponsored-by: TrueNAS
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@truenas.com>
Closes #18296

cmd/zinject/translate.c

@@ -85,7 +85,7 @@ static int
 parse_pathname(const char *inpath, char *dataset, char *relpath,
     struct stat64 *statbuf)
 {
-	struct extmnttab mp;
+	struct mnttab mp;
 	const char *rel;
 	char fullpath[MAXPATHLEN];