savecore(8): Fix buffer overrun inspecting disks with varying sector size

[cem]

Summary

A premature optimization lead to caching a native-sector sized memory
allocation. If the program examined a 512 byte sector disk, then a 4096
byte sector disk, the program would overrun the cached 512 byte buffer.

Just remove the optimization to fix the bug.

Noticed by: markj

[rpokala]

'temp' is the buffer you read the header into, and also what you use to write the "cleared" header. So what's 'buf'?

[cem]

In D8162#169285, @rpokala wrote:

'temp' is the buffer you read the header into, and also what you use to write the "cleared" header. So what's 'buf'?

temp is used for kernel dump metadata and is only a single sector large. buf is used for streaming dump data off the disk, and is 1MB.

[markj]

Thanks!