f2fs: fix typo

Fix typo and some grammatical errors.

The words "filesystem" and "readahead" are being used without the space treewide.

Signed-off-by: Park Ju Hyung <qkrwngud825@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 657582f..633315a 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -663,7 +663,7 @@
 	if (need_restart_gc) {
 		if (start_gc_thread(sbi))
 			f2fs_msg(sbi->sb, KERN_WARNING,
-				"background gc thread is stop");
+				"background gc thread has stopped");
 	} else if (need_stop_gc) {
 		stop_gc_thread(sbi);
 	}