initramfs/initcpio: create MOUNT_FOLDER if it doesn't exist

/usr/lib/kdumpst/kdump-mkinitcpio-hook.sh is run by the kernel hook
when using mkinitcpio and a new kernel is installed.

If MOUNT_FOLDER does not exist it will fail with the following error
message:

==> ERROR: Invalid option -g -- '<MOUNT_FOLDER>/<IMG_FILE>' must be writable

initramfs/initcpio/initcpio-common.sh

@@ -6,6 +6,7 @@
 #  before running any of these functions!
 #
 create_initramfs_mkinitcpio() {
+	mkdir -p "${MOUNT_FOLDER}"
 	rm -f "${MOUNT_FOLDER}/kdump-initrd-$1.img"
 
 	mkinitcpio -A kdump -S microcode,kms,plymouth -g "${MOUNT_FOLDER}/kdump-initrd-$1.img" -k "$1" 1>/dev/null