[PATCH 9_9] Use "void *" not "unsigned long *" for block dev read_write buffer pointers

[PATCH 9_9] Use "void *" not "unsigned long *" for block dev read_write buffer pointers

Block device read/write is anonymous data; there is no need to use a
typed pointer.  void * is fine.  Also add a hook for block_read functions

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>