linux: Move posix dir implementations to Linux

This generic implementation already expects a getdents API which
is Linux specific.  It also allows simplify it by assuming
_DIRENT_HAVE_D_RECLEN and _DIRENT_HAVE_D_OFF support.

The readdir are also expanded on each required implementation,
futher fixes and improvements will make parametrize the
implementation more complex.

Checked on x86_64-linux-gnu, i686-linux-gnu, and with a build
for all affected ABIs.

sysdeps/unix/sysv/linux/readdir.c

@@ -19,5 +19,68 @@
 #include <dirent.h>
 
 #if !_DIRENT_MATCHES_DIRENT64
-# include <sysdeps/posix/readdir.c>
+#include <dirstream.h>
+
+/* Read a directory entry from DIRP.  */
+struct dirent *
+__readdir (DIR *dirp)
+{
+  struct dirent *dp;
+  int saved_errno = errno;
+
+#if IS_IN (libc)
+  __libc_lock_lock (dirp->lock);
+#endif
+
+  do
+    {
+      size_t reclen;
+
+      if (dirp->offset >= dirp->size)
+	{
+	  /* We've emptied out our buffer.  Refill it.  */
+
+	  size_t maxread = dirp->allocation;
+	  ssize_t bytes;
+
+	  bytes = __getdents (dirp->fd, dirp->data, maxread);
+	  if (bytes <= 0)
+	    {
+	      /* On some systems getdents fails with ENOENT when the
+		 open directory has been rmdir'd already.  POSIX.1
+		 requires that we treat this condition like normal EOF.  */
+	      if (bytes < 0 && errno == ENOENT)
+		bytes = 0;
+
+	      /* Don't modifiy errno when reaching EOF.  */
+	      if (bytes == 0)
+		__set_errno (saved_errno);
+	      dp = NULL;
+	      break;
+	    }
+	  dirp->size = (size_t) bytes;
+
+	  /* Reset the offset into the buffer.  */
+	  dirp->offset = 0;
+	}
+
+      dp = (struct dirent *) &dirp->data[dirp->offset];
+
+      reclen = dp->d_reclen;
+
+      dirp->offset += reclen;
+
+      dirp->filepos = dp->d_off;
+
+      /* Skip deleted files.  */
+    } while (dp->d_ino == 0);
+
+#if IS_IN (libc)
+  __libc_lock_unlock (dirp->lock);
+#endif
+
+  return dp;
+}
+weak_alias (__readdir, readdir)
+
 #endif