libxfs: sanitize agcount on load

libxfs: sanitize agcount on load

Before we get into libxfs_initialize_perag and try to blindly
allocate a perag struct for every (possibly corrupted number of)
AGs, see if we can read the last one.  If not, assume it's corrupt,
and load only the first AG.

Do this only for an arbitrarily high-ish agcount, so that normal-ish
geometry on a possibly truncated file or device will still do
its best to make all readable AGs available.

Set xfs_db's exitcode to 1 if this happens.

Also teach metadump to detect this and exit appropriately if
truncated, as it resets exitcode to 0 for its own purposes internally.

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