bcachefs: Improve error message when finding wrong btree node

single_device.merge_torture_flakey is, very rarely, finding a btree node
that doesn't match the key that points to it: this patch improves the
error message to print out more fields from the btree node header, so
that we can see what else does or does not match the key.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>