f2fs: monitor the number of background checkpoint

This patch adds to show the number of background checkpoint.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index d8ad1ab..5904a41 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -297,6 +297,7 @@
 		if (test_opt(sbi, DATA_FLUSH))
 			sync_dirty_inodes(sbi, FILE_INODE);
 		f2fs_sync_fs(sbi->sb, true);
+		stat_inc_bg_cp_count(sbi->stat_info);
 	}
 }