mount.nilfs2: fix ignored error on remount target mismatch

In nilfs_prepare_mount(), when a rw->rw remount is attempted on a
different mount point, an error message is printed, but the function
fails to abort the process.

This occurs because the return variable 'res' remains 0 (success) when
jumping to the failure label.

While a regular mount would eventually fail in the system call, a fake
mount (-f) skips the system call and would incorrectly succeed with exit
code 0 despite the error.

Fix this by setting 'res' to -EINVAL.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>

sbin/mount/mount_libmount.c

@@ -358,6 +358,7 @@ static int nilfs_prepare_mount(struct nilfs_mount_info *mi)
 					 mnt_table_get_cache(mtab))) {
 			error(_("%s: different mount point (%s). remount failed."),
 			      progname, mnt_context_get_target(cxt));
+			res = -EINVAL;
 			goto failed;
 		}