fstab-generator: if we mount via roothash=/usrhash= let's imply "ro" mount option

fstab-generator: if we mount via roothash=/usrhash= let's imply "ro" mount option

If we discover the root or /usr/ fs via roothash=/usrhash= we know the
file system mounted on it will be read-only, since Verity volumes are by
definition immutable. Hence, let's imply the "ro" mount option for them.

This way the "kernel: /dev/mapper/usr: Can't open blockdev" boot-time
log message goes away, reported here:

https://github.com/systemd/systemd/issues/27682

(I do wonder though why erofs even tries to open the block device as
writable, that sounds utterly pointless for a file system that carries
the fact it is read-only even in the name...)