fs: add hostfs/rpmsgfs link/symlink/readlink/lstat backends

[Kaben123]

Summary

Implement link, symlink, readlink, and lstat support for the hostfs and rpmsgfs filesystem backends, utilizing the mountpt_operations interfaces added in #19949.

This is PR#3 of a 4-part PSE52/POSIX filesystem enhancement series:

• ✅ PR#1 — link/mountpt infrastructure (fs: add hardlink and mountpt link/lstat infrastructure #19949, merged)
• ✅ PR#2 — inode path-resolution infrastructure (fs/inode: add POSIX path-resolution infrastructure #19950, merged)
• PR#3 — filesystem backends (this PR)
• PR#4 — target bug fixes (pending PR#3 merge)

Changes (4 commits)

1. fs/hostfs: move global lock into hostfs_mountpt_s — Replace the global g_lock with a per-filesystem fs->fs_lock, improving concurrency for multi-mount scenarios.

2. fs/hostfs: change fs_heap to lib_tempbuffer — Migrate hostfs path buffer allocation from fs_heap/stack arrays to the lib_get_tempbuffer/lib_put_tempbuffer system, aligning with the inode layer (merged in fs/inode: add POSIX path-resolution infrastructure #19950).

3. fs/hostfs: add link, symlink, readlink and lstat support — Implement the four operations for hostfs, including arch host stubs for sim/arm/arm64/risc-v/xtensa. Adds the necessary nuttx_host_link, nuttx_host_symlink, nuttx_host_readlink, and nuttx_host_lstat host-side functions.

4. fs/rpmsgfs: add link, symlink, readlink and lstat support — Implement the four operations for rpmsgfs, adding the corresponding client RPC handlers and server-side dispatch.

Testing

• sim:nsh with CONFIG_FS_HOSTFS=y + CONFIG_FS_LINKS=y: builds cleanly, zero errors, zero warnings.

Impact

• 14 files changed, +1155/−129 lines
• No breaking changes — new operations are added at the end of the ops table with NULL placeholders in unaffected filesystems

[github-actions]

MemBrowse Memory Report

qemu-armv8a

• Code: .text.hostfs_bind +32 B, .text.hostfs_chstat +40 B, .text.hostfs_close -4 B, .text.hostfs_closedir -4 B, .text.hostfs_fchstat -4 B, .text.hostfs_fstat -4 B, .text.hostfs_ftruncate -4 B, .text.hostfs_ioctl -4 B, .text.hostfs_mkdir +32 B, .text.hostfs_open +28 B, .text.hostfs_opendir +48 B, .text.hostfs_read -4 B, .text.hostfs_readdir -4 B, .text.hostfs_rename +64 B, .text.hostfs_rewinddir -4 B, .text.hostfs_rmdir +40 B, .text.hostfs_seek -4 B, .text.hostfs_stat +32 B, .text.hostfs_statfs -4 B, .text.hostfs_sync -4 B, .text.hostfs_unbind +4 B, .text.hostfs_unlink +40 B, .text.hostfs_write -4 B (+0.1%, 319,712 B)
• Data: -24 B (-0.0%, 76,953 B)
  No memory changes detected for:
• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb