xfs: add the ability to flag a fs for repair

This "new feature" adds a new incompat feature flag so that we can force
a sysadmin to run xfs_repair on a filesystem before mounting.  The
intent for this code is to make it so that one can use xfs_db to upgrade
a filesystem to support new V5 features (e.g. y2038 or inode btree
counters).  Because some upgrades may require xfs_repair to fix or add
things before the filesystem goes back into use, this is the means for
xfs_db to force that to happen.

Note: xfs_admin will automatically run repair when required, so
sysadmins won't have to issue the repair command directly.

v2: move all the kernel-specific checks to xfs_fs_fill_super since that's
where we put the rest of them already