Print symbol index for unsupported relocation types

It is unlikely, but possible, that an unrecognized or unsupported
relocation type is encountered while trying to load a kernel module. If
this occurs we should offer the symbol index as a hint to the user.

While here, fix some small style issues.

Reviewed by: markj, kib (amd64 part, in D26701)
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.