lib: posix: update option group of readdir_r()

`readdir_r()` belongs to the following option group

POSIX_FILE_SYSTEM_R: Thread-Safe File System

Create a new Kconfig `CONFIG_POSIX_FILE_SYSTEM_R` to compile
it.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>

Author: ycsin

lib/posix/options/Kconfig.fs

@@ -17,4 +17,13 @@ config POSIX_FILE_SYSTEM_ALIAS_FSTAT
 	help
 	  Select 'y' here and Zephyr will provide an alias for fstat() as _fstat().
 
+config POSIX_FILE_SYSTEM_R
+	bool "Thread-Safe File System"
+	help
+	  Select 'y' here and Zephyr will provide an implementation of the POSIX_FILE_SYSTEM_R
+	  Option Group, consisting of readdir_r().
+
+	  For more informnation, please see
+	  https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_subprofiles.html
+
 endif # POSIX_FILE_SYSTEM

lib/posix/options/Kconfig.pthread

@@ -156,6 +156,7 @@ config POSIX_THREAD_PRIO_PROTECT
 
 config POSIX_THREAD_SAFE_FUNCTIONS
 	bool "POSIX thread-safe functions"
+	select POSIX_FILE_SYSTEM_R if POSIX_FILE_SYSTEM
 	help
 	  Select 'y' here to enable POSIX thread-safe functions including asctime_r(), ctime_r(),
 	  flockfile(), ftrylockfile(), funlockfile(), getc_unlocked(), getchar_unlocked(),

lib/posix/options/fs.c

@@ -314,7 +314,7 @@ struct dirent *readdir(DIR *dirp)
 	return &pdirent;
 }
 
-#ifdef CONFIG_POSIX_THREAD_SAFE_FUNCTIONS
+#ifdef CONFIG_POSIX_FILE_SYSTEM_R
 int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result)
 {
 	struct dirent *dir;
@@ -346,7 +346,7 @@ int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result)
 
 	return 0;
 }
-#endif /* CONFIG_POSIX_THREAD_SAFE_FUNCTIONS */
+#endif /* CONFIG_POSIX_FILE_SYSTEM_R */
 
 /**
  * @brief Rename a file.