explicitly detach loop device on umount and silent losetup error

Umount detaches the loop device automatically, but let's make it
explicit to be sure. Additionally losetup gives:

losetup: /dev/loop0: detach failed: No such device or address

This is kind of expected, let's silent the error message.

Signed-off-by: Christian Hesse <mail@eworm.de>

archiso/initcpio/hooks/archiso

@@ -182,7 +182,7 @@ archiso_mount_handler() {
     fi
 
     if [[ "${copytoram}" == "y" ]]; then
-        umount /run/archiso/bootmnt
+        umount -d /run/archiso/bootmnt
     fi
 }
 

archiso/initcpio/hooks/archiso_loop_mnt

@@ -26,7 +26,7 @@ archiso_loop_mount_handler () {
     archiso_mount_handler ${newroot}
 
     if [[ "${copytoram}" == "y" ]]; then
-        losetup -d ${_dev_loop}
+        losetup -d ${_dev_loop} 2>/dev/null
         umount /run/archiso/img_dev
     fi
 }