Fixes for bugs not shared with ffs:
- don't mount unclean filesystems rw unless forced to.
- accept EXT2_ERROR_FS (treat it like !EXT2_VALID_FS). We still don't set
this or honour the maximal mount count.
- don't attempt to print the name of the mount point when mounting an
unclean file system, since the name of the previous mount point is
unknown and the name of the current mount point is still "".
Fixes for bugs shared with ffs until recently:
- don't set the clean flag on unmount of an initially-unclean filesystem
that was (forcibly) mounted rw.
- set the clean flag on rw -> ro update of a mounted initially-clean
filesystem.
- fixed some style bugs (mostly long lines).
The fixes are slightly simpler than for ffs, because the relevant on-disk
state is not a simple boolean variable, and the superblock has a core-only
extension.
Obtained from: parts from ffs_vfsops.c, parts from NetBSD
687ba8c231