pid1: improve logging when failing to remount / ro (#5940)

pid1: improve logging when failing to remount / ro (#5940)

https://bugzilla.redhat.com/show_bug.cgi?id=1227736#c49

We counted how many filesystems could not be unmounted, but only for those
filesystems which we tried to unmount. Since we only remount / ro, without
attempting to unmount, we would emit a confusing error message:

Remounting '/' read-only with options 'seclabel,space_cache,subvolid=5,subvol=/'.
Remounting '/' read-only with options 'seclabel,space_cache,subvolid=5,subvol=/'.
Remounting '/' read-only with options 'seclabel,space_cache,subvolid=5,subvol=/'.
All filesystems unmounted.

Warn when remount-ro fails, and for filesystems which we won't try to unmount,
include the failure to remount-ro in n_failed.

A few minor cleanups:
- remove unecessary goto which jumps to the next line anyway
- always calculate n_failed, even if log_error is false. This causes no change
  in behaviour, but I think the code is easier to follow, since the log setting
  cannot influence other logic.