metadump: bounds check btree block regions being zeroed

metadump: bounds check btree block regions being zeroed

Arkadiusz Miskiewicz reported that metadump was crashing on one of
his corrupted filesystems, and the trace indicated that it was
zeroing unused regions in inode btree blocks when it failed. The
btree block had a corrupt nrecs field, which was resulting in an out
of bounds memset() occurring.  Ensure that the region being
generated for zeroing is within bounds before executing the zeroing.

Reported-by: Arkadiusz Miskiewicz <arekm@maven.pl>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>