The goal of this change is to prevent accidental foot shooting by
folks running filesystems created on check-hash enabled kernels
(which I will call "new") on a non-check-hash enabled kernels (which
I will call "old). The idea here is to detect when a filesystem is
run on an old kernel and flag the filesystem so that when it gets
moved back to a new kernel, it will not start getting a slew of
check-hash errors.

Back when the UFS version 2 filesystem was created, it added a file
flag FS_INDEXDIRS that was to be set on any filesystem that kept
some sort of on-disk indexing for directories. The idea was precisely
to solve the issue we have today. Specifically that a newer kernel
that supported indexing would be able to tell that the filesystem
had been run on an older non-indexing kernel and that the indexes
should not be used until they had been rebuilt. Since we have never
implemented on-disk directory indicies, the FS_INDEXDIRS flag is
cleared every time any UFS version 2 filesystem ever created is
mounted for writing.

This commit repurposes the FS_INDEXDIRS flag as the FS_METACKHASH
flag. Thus, the FS_METACKHASH is definitively known to have always
been cleared. The FS_INDEXDIRS flag has been moved to a new block
of flags that will always be cleared starting with this commit
(until they get used to implement some future feature which needs
to detect that the filesystem was mounted on a kernel that predates
the new feature).

If a filesystem with check-hashes enabled is mounted on an old
kernel the FS_METACKHASH flag is cleared. When that filesystem is
mounted on a new kernel it will see that the FS_METACKHASH has been
cleared and clears all of the fs_metackhash flags. To get them
re-enabled the user must run fsck (in interactive mode without the
-y flag) which will ask for each supported check hash whether it
should be rebuilt and enabled. When fsck is run in its default preen
mode, it will just ignore the check hashes so they will remain
disabled.

The kernel has always disabled any check hash functions that it
does not support, so as more types of check hashes are added, we
will get a non-surprising result. Specifically if filesystems get
moved to kernels supporting fewer of the check hashes, those that
are not supported will be disabled. If the filesystem is moved back
to a kernel with more of the check-hashes available and fsck is run
interactively to rebuild them, then their checking will resume.
Otherwise just the smaller subset will be checked.

A side effect of this commit is that filesystems running with
cylinder-group check hashes will stop having them checked until
fsck is run to re-enable them (since none of them currently have
the FS_METACKHASH flag set). So, if you want check hashes enabled
on your filesystems after booting a kernel with these changes, you
need to run fsck to enable them. Any newly created filesystems will
have check hashes enabled. If in doubt as to whether you have check
hashes emabled, run dumpfs and look at the list of enabled flags
at the end of the superblock details.