ext4 / jbd2: add fast commit initialization

This patch adds fast commit area trackers in the journal_t
structure. These are initialized via the jbd2_fc_init() routine that
this patch adds. This patch also adds ext4/fast_commit.c and
ext4/fast_commit.h files for fast commit code that will be added in
subsequent patches in this series.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201015203802.3597742-4-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/fast_commit.c

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * fs/ext4/fast_commit.c
+ *
+ * Written by Harshad Shirwadkar <harshadshirwadkar@gmail.com>
+ *
+ * Ext4 fast commits routines.
+ */
+#include "ext4_jbd2.h"
+
+void ext4_fc_init(struct super_block *sb, journal_t *journal)
+{
+	if (!test_opt2(sb, JOURNAL_FAST_COMMIT))
+		return;
+	if (jbd2_fc_init(journal, EXT4_NUM_FC_BLKS)) {
+		pr_warn("Error while enabling fast commits, turning off.");
+		ext4_clear_feature_fast_commit(sb);
+	}
+}