Move malloc_large_slab and malloc_large_size to malloc.h

[kvinupriya_gmail.com]

Summary

These functions are useful for code that wants to get sizes for a
malloc type.

Sponsored by: Juniper Networks, Inc.

[khng]

Or you probably want to expose a proper API rather?

[markj]

These functions are useful for code that wants to get sizes for a malloc type.

This is perhaps addressed by commit 1c30cf95098e999ee24b2396763b4f65d021cdf0?

[khng]

Or that can be addressed by malloc_usable_size() ?

[markj]

In D45282#1056094, @khng wrote:

Or that can be addressed by malloc_usable_size() ?

Potentially yes, depending on the details of what you are doing.

In any case, this patch is probably not the right way to go. If there is some missing malloc(9) interface, we should add it rather than exposing implementation details.