Optimize operations on UFS/FFS filesystems with bad cylinder group(s).

If a UFS/FFS filesystem develops a broken cylinder group (which is
usually detected when its check hash fails), that cylinder group
will not be usable until the filesystem has been unmounted and fsck
has been run to repair it. On the first attempt to to allocate
resources from the broken cylinder group, its available resources
are set to zero in the superblock summary information. Since it
will appear to have no resources available, no further calls will
be made to allocate resources from it. When resources are freed to
the broken cylinder group, the resource free routines will find the
cylinder group unusable so the resource will simply be discarded
and thus will not show up in the superblock summary information
until they are recovered by fsck.

Reported-by: Peter Holm
Tested-by: Peter Holm
MFC-after: 1 week
Sponsored-by: The FreeBSD Foundation