report what the bad first instruction actually is

Not sure why I'm getting this:

`/usr/local/libexec/kpatch/create-diff-object: kpatch_create_mcount_sections: 2343: bad first instruction in generic_write_checks`

But it doesn't actually say what the bad instruction is. It'll now report:

`/usr/local/libexec/kpatch/create-diff-object: kpatch_create_mcount_sections: 2343: bad first instruction '0x55' in generic_write_checks, should be 0xf`

For what it's worth, this is a simple function call added to `generic_write_checks` out of `mm/filemap.c`