[LibOS] Replace linked list of allowed_files with something more performant

[dimakuv]

Description of the feature

(Extracted during review of #1812)

@mwkmwkmwk wrote:

I am a little concerned about stuffing all created files in the allowed_files list — the list has quadratic behavior and if an application creates a fair amount of files, this is going to hurt. should we think about alternatives?

@dimakuv wrote:

I can imagine the following optimization:

• Split "allowed files" internally into a linked list of allowed dirs and a hash table of allowed files
• On open-file checks, first traverse the list of allowed dirs -- this is fast because the list is typically minimal, 10-20 items max
• Then check the hash table of allowed files
• On creating files, add the file to the hash table of allowed files (i.e. never to the list of allowed dirs)

Why Gramine should implement it?

Improve performance of allowed files checks.