xfs: deprecate the V4 format

The V4 filesystem format contains known weaknesses in the on-disk format
that make metadata verification diffiult.  In addition, the format does
not support dates past 2038 and will not be upgraded to do so.  We
should start the process of retiring the old format to close off attack
surfaces and to encourage users to migrate onto V5.

Therefore, make XFS V4 support a configurable option.  For the first
period it will be default Y in case some distributors want to withdraw
support early; for the second period it will be default N so that anyone
who wishes to continue support can do so; and after that, support will
be removed from the kernel.  Dates for these events have been added to
the upstream kernel.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>