Fix for re-reading /etc/mtab in zfs_is_mounted()

When /etc/mtab is updated on Linux it's done atomically with
rename(2).  A new mtab is written, the existing mtab is unlinked,
and the new mtab is renamed to /etc/mtab.  This means that we
must close the old file and open the new file to get the updated
contents.  Using rewind(3) will just move the file pointer back
to the start of the file, freopen(3) will close and open the file.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#1611

lib/libzfs/libzfs_dataset.c

@@ -629,12 +629,15 @@ libzfs_mnttab_init(libzfs_handle_t *hdl)
 	    sizeof (mnttab_node_t), offsetof(mnttab_node_t, mtn_node));
 }
 
-void
+int
 libzfs_mnttab_update(libzfs_handle_t *hdl)
 {
 	struct mnttab entry;
 
-	rewind(hdl->libzfs_mnttab);
+	/* Reopen MNTTAB to prevent reading stale data from open file */
+	if (freopen(MNTTAB, "r", hdl->libzfs_mnttab) == NULL)
+		return (ENOENT);
+
 	while (getmntent(hdl->libzfs_mnttab, &entry) == 0) {
 		mnttab_node_t *mtn;
 
@@ -647,6 +650,8 @@ libzfs_mnttab_update(libzfs_handle_t *hdl)
 		mtn->mtn_mt.mnt_mntopts = zfs_strdup(hdl, entry.mnt_mntopts);
 		avl_add(&hdl->libzfs_mnttab_cache, mtn);
 	}
+
+	return (0);
 }
 
 void
@@ -677,13 +682,18 @@ libzfs_mnttab_find(libzfs_handle_t *hdl, const char *fsname,
 {
 	mnttab_node_t find;
 	mnttab_node_t *mtn;
+	int error;
 
 	if (!hdl->libzfs_mnttab_enable) {
 		struct mnttab srch = { 0 };
 
 		if (avl_numnodes(&hdl->libzfs_mnttab_cache))
 			libzfs_mnttab_fini(hdl);
-		rewind(hdl->libzfs_mnttab);
+
+		/* Reopen MNTTAB to prevent reading stale data from open file */
+		if (freopen(MNTTAB, "r", hdl->libzfs_mnttab) == NULL)
+			return (ENOENT);
+
 		srch.mnt_special = (char *)fsname;
 		srch.mnt_fstype = MNTTYPE_ZFS;
 		if (getmntany(hdl->libzfs_mnttab, entry, &srch) == 0)
@@ -693,7 +703,8 @@ libzfs_mnttab_find(libzfs_handle_t *hdl, const char *fsname,
 	}
 
 	if (avl_numnodes(&hdl->libzfs_mnttab_cache) == 0)
-		libzfs_mnttab_update(hdl);
+		if ((error = libzfs_mnttab_update(hdl)) != 0)
+			return (error);
 
 	find.mtn_mt.mnt_special = (char *)fsname;
 	mtn = avl_find(&hdl->libzfs_mnttab_cache, &find, NULL);