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/segment.h b/fs/f2fs/segment.h
index 55973f7..ff48325 100644
--- a/fs/f2fs/segment.h
+++ b/fs/f2fs/segment.h
@@ -549,7 +549,7 @@
 }
 
 /*
- * Summary block is always treated as invalid block
+ * Summary block is always treated as an invalid block
  */
 static inline void check_block_count(struct f2fs_sb_info *sbi,
 		int segno, struct f2fs_sit_entry *raw_sit)