nuttx fs_heap: added support for initialization from a specified section

Summary

Why is this change necessary? This PR is to enhance the use of fs_heap.

Changes

Based on the changes in https://github.com/apache/nuttx/pull/13150, I added a new way to initialize fs_heap by allocating memory through the specified section. This method requires modifying the ld file to allocate a space for fs to use
Replace all kmm_ application codes in nuttx/fs with fs_heap. By default, it is equivalent to kmm_
Move accept4 from net to fs because it uses sockfd_allocate, which will cause its priv to be released in fs.

Impact

Is a new feature added?: NO Impact on build: NO Impact on hardware: NO, this change does not specifically target any particular hardware architectures or boards. Impact on documentation: NO,This patch does not introduce any new features Impact on compatibility: This implementation aims to be backward compatible. No existing functionality is expected to be broken.

Testing

Build Host(s): Linux x86 Target(s): sim/nsh Based on the relevant fs case use case is passed (Because fs_heap_ is equivalent to kmm_ by default, there is no impact on any existing configurations.)