xfs: be more forgiving of a v4 secondary sb w/ junk in v5 fields

Today, if xfs_sb_read_verify encounters a v4 superblock
with junk past v4 fields which includes data in sb_crc,
it will be treated as a failing checksum and a significant
corruption.

There are known prior bugs which leave junk at the end
of the V4 superblock; we don't need to actually fail the
verification in this case if other checks pan out ok.

So if this is a secondary superblock, and the primary
superblock doesn't indicate that this is a V5 filesystem,
don't treat this as an actual checksum failure.

We should probably check the garbage condition as
we do in xfs_repair, and possibly warn about it
or self-heal, but that's a different scope of work.

Stable folks: This can go back to v3.10, which is what
introduced the sb CRC checking that is tripped up by old,
stale, incorrect V4 superblocks w/ unzeroed bits.

Cc: [email protected]
Signed-off-by: Eric Sandeen <[email protected]>
Acked-by: Dave Chinner <[email protected]>
Reviewed-by: Mark Tinguely <[email protected]>
Signed-off-by: Ben Myers <[email protected]>

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