initrd: Use str_plural() in rd_load_image()

Add the local variable 'nr_disks' and replace the manual ternary "s"
pluralization with the standardized str_plural() helper function.

Use pr_notice() instead of printk(KERN_NOTICE) to silence a checkpatch
warning.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>