DEBUG: pools: report the data around the offending area in case of mismatch

DEBUG: pools: report the data around the offending area in case of mismatch

When the integrity check fails, it's useful to get a dump of the area
around the first faulty byte. That's what this patch does. For example
it now shows this before reporting info about the tag itself:

  Contents around first corrupted address relative to pool item:.
  Contents around address 0xe4febc0792c0+40=0xe4febc0792e8:
    0xe4febc0792c8 [80 75 56 d8 fe e4 00 00] [.uV.....]
    0xe4febc0792d0 [a0 f7 23 a4 fe e4 00 00] [..#.....]
    0xe4febc0792d8 [90 75 56 d8 fe e4 00 00] [.uV.....]
    0xe4febc0792e0 [d9 93 fb ff fd ff ff ff] [........]
    0xe4febc0792e8 [d9 93 fb ff ff ff ff ff] [........]
    0xe4febc0792f0 [d9 93 fb ff ff ff ff ff] [........]
    0xe4febc0792f8 [d9 93 fb ff ff ff ff ff] [........]
    0xe4febc079300 [d9 93 fb ff ff ff ff ff] [........]

This may be backported to 2.9 and maybe even 2.8 as it does help spot
the cause of the memory corruption.