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/super.c b/fs/nilfs2/super.c
index 92579cc..896b463 100644
--- a/fs/nilfs2/super.c
+++ b/fs/nilfs2/super.c
@@ -436,7 +436,7 @@
 	/*
 	 * Compute the overhead
 	 *
-	 * When distributing meta data blocks outside semgent structure,
+	 * When distributing meta data blocks outside segment structure,
 	 * We must count them as the overhead.
 	 */
 	overhead = 0;