libblkid: prune unneeded buffers

libblkid: prune unneeded buffers

When a new buffer is cached that is a superset of another existing
buffer the old buffer can be removed as future requests can be satisfied
by the new one.

As probe functions can have local references to buffered data, delay the
cleanup until the probefunc is finished to avoid accessing freed data.

For the bcachefs.img testfile this reduces the final (maximal) cache
from 34338 bytes in 54 buffers
  to 24760 bytes in 40 buffers.

Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>