db/log_reader: handle modified CRCs

db/log_reader: handle modified CRCs

If we detect a CRC error on the first record of a log file, check to see
whether the CRC has the log_number mixed in.  If so, we set the recycled_
flag, and thereafter assume that the CRC will have log_number mixed in.
This raises our probability of a false-positive CRC to 2/2^32 only
for the first record, but leaves it at 1/2^32 for the rest of each file.

Signed-off-by: Sage Weil <sage@redhat.com>