Introduce a utility function for moving to the next dirent in read_dir.

This takes care of making sure the dirent buffer is properly aligned,
which it needs to be on some platforms (SPARC, ARM, etc.)

Change-Id: I9a6352b1e654c090a200770d51f96511ee024a99

headers/private/file_systems/fs_ops_support.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2022, Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef FS_OPS_SUPPORT_H
+#define FS_OPS_SUPPORT_H
+
+#ifndef FS_SHELL
+#	include <kernel.h>
+#	include <kernel/debug.h>
+#	include <dirent.h>
+#else
+#	include "fssh_kernel_priv.h"
+#endif
+
+
+static struct dirent*
+next_dirent(struct dirent* dirent, size_t nameLength, size_t& bufferRemaining)
+{
+	const size_t reclen = offsetof(struct dirent, d_name) + nameLength + 1;
+	ASSERT(reclen <= bufferRemaining);
+	dirent->d_reclen = reclen;
+
+	const size_t roundedReclen = ROUNDUP(reclen, alignof(struct dirent));
+	if (roundedReclen >= bufferRemaining) {
+		bufferRemaining -= reclen;
+		return NULL;
+	}
+	dirent->d_reclen = roundedReclen;
+	bufferRemaining -= roundedReclen;
+
+	return (struct dirent*)((uint8*)dirent + roundedReclen);
+}
+
+
+#endif	// FS_OPS_SUPPORT_H