xfs: strengthen log intent validation

This patchset hoists the code that checks log intent record validation
into separate functions, and reworks them to use the standard field
validation predicates instead of open-coding them.  This strengthens log
recovery against (some) fuzzed log items.

v2: rearrange some of the checks per hch; report intent item corruption
v3: call XFS_CORRUPTION_ERROR to dump the bad intent item to dmesg