nilfs2: fix various typos in comments

This fixes various typos I found in comments of nilfs2.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/dat.c b/fs/nilfs2/dat.c
index 9d1e5de..0131467 100644
--- a/fs/nilfs2/dat.c
+++ b/fs/nilfs2/dat.c
@@ -288,7 +288,7 @@
  * @vblocknrs and @nitems.
  *
  * Return Value: On success, 0 is returned. On error, one of the following
- * nagative error codes is returned.
+ * negative error codes is returned.
  *
  * %-EIO - I/O error.
  *