Disable zfs /etc/mtab updates

Completely disable the zfs binary from attempting to directly update
/etc/mtab.  The Linux port relies entirely on the mount.zfs helper
to safely update /etc/mtab.  If we left the /etc/mtab updates to
the zfs binary then they could race with concurrent non-zfs mounts.
Routing everything through the system mount command ensures the
/etc/mtab updates are locked properly.

Signed-off-by: Brian Behlendorf <[email protected]>
Issue openzfs#329

cmd/zfs/zfs_main.c

@@ -6177,7 +6177,7 @@ main(int argc, char **argv)
 	/*
 	 * Run the appropriate command.
 	 */
-	libzfs_mnttab_cache(g_zfs, B_TRUE);
+	libzfs_mnttab_cache(g_zfs, B_FALSE);
 	if (find_command_idx(cmdname, &i) == 0) {
 		current_command = &command_table[i];
 		ret = command_table[i].func(argc - 1, argv + 1);
@@ -6191,7 +6191,6 @@ main(int argc, char **argv)
 		usage(B_FALSE);
 		ret = 1;
 	}
-	libzfs_mnttab_cache(g_zfs, B_FALSE);
 	libzfs_fini(g_zfs);
 
 	(void) fclose(mnttab_file);