[LibOS] Keep all VMAs of a given LibOS handle on a linked list

[dimakuv]

Description of the feature

Currently, if some LibOS handle operation needs to access a VMA corresponding to that memory address/range, the only way to do so is to iterate over all VMAs and filter out the ones that correspond to this LibOS handle. Thus, even if the LibOS handle a couple VMAs associated with it, any VMA-accessing operation will see traverse over all VMAs. This can become expensive.

One example is reload_mmaped_from_file_handle(). Another example can be found in PR #1818.

The proposal from @mwkmwkmwk is to keep all VMAs of a given libos_handle on a linked list. This will significantly speed up VMA-specific operations linked to a particular handle.

This issue is extracted from discussion https://reviewable.io/reviews/gramineproject/gramine/1818#-NybRQD02P8LkpEw7ztX

Why Gramine should implement it?

Performance.