Refactor path resolution

Currently, path resolution is using lstat() to ensure that each path component exists. This can add unnecessary overhead, especially when the mount point is located under a long path within a parallel file system like Lustre.

The requirements for path checking:

1. Detect paths within GekkoFS and pass them (without prefix) to GekkoFS
2. No system calls for checking of path component existence.
3. When a path is not within GekkoFS, the unmodified path is passed to the kernel, except a GekkoFS path is part of the path which must be removed first.

Therefore, the following features/changes are necessary:

• A prefix is a defined as the absolute path to the GekkoFS mountpoint. E.g., for /tmp/gkfs_mount/foofile the prefix is /tmp/gkfs_mount.
• No system call like lstat() should be called
• Path checking is done by prefix matching. Therefore a non-absolute path first needs to be resolved:
  • if the middle of the path uses .., the path needs to be resolved for prefix checking (similar to now)
  • relative paths (based on the current working directory) also need to be resolved for prefix checking
• If a path is not within GekkoFS, the path should be passed to the kernel unmodified.
  • When part of the GekkoFS namespace is in the middle of a path and then undone via .., the path needs to be resolved before passing it to the kernel as the kernel is unaware of GekkoFS
  • This also applies to relative paths
• If a path is within GekkoFS, the prefix is cut from the path and passed to GekkoFS.

Part of these improvements are included but unfinished in this branch