mount.nilfs2: fix broken overlapping rw-mount protection

The check for overlapping rw-mounts in nilfs_prepare_mount() detects
the condition and prints an error message, but fails to stop the mount
process.

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

Consequently, the mount succeeds illegally, leading to a state where
umount fails with -EBUSY (due to unhandled garbage collector shutdown),
forcing manual process termination.

Fix this by setting 'res' to -EBUSY.  While any negative value would
abort the mount, -EBUSY is chosen to accurately reflect the cause and
align with libmount's error reporting granularity.

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

sbin/mount/mount_libmount.c

@@ -334,6 +334,7 @@ static int nilfs_prepare_mount(struct nilfs_mount_info *mi)
 		error(_("%s: the device already has a rw-mount on %s.\n"
 			"\t\tmultiple rw-mount is not allowed."),
 		      progname, mnt_fs_get_target(fs));
+		res = -EBUSY;
 		goto failed;
 	case MS_RDONLY: /* ro-mount (a rw-mount exists) */
 		break;