Directly allocate buffer memory in a pvck scan instead of using a mempool.

Directly allocate buffer memory in a pvck scan instead of using a mempool.

There's a very high memory usage when calling _pv_analyse_mda_raw (e.g. while
executing pvck) that can end up with "out of memory".

_pv_analyse_mda_raw scans for metadata in the MDA, iteratively increasing the
size to scan with SECTOR_SIZE until we find a probable config section or we're
at the edge of the metadata area. However, when using a memory pool, we're also
iteratively chasing for bigger and bigger mempool chunk which can't be found
and so we're always allocating a new one, consuming more and more memory...

This patch just changes the mempool to direct memory allocation in this
problematic part of the code.