Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
diff --git a/include/linux/reiserfs_fs.h b/include/linux/reiserfs_fs.h
index 0a3842a..eca75df 100644
--- a/include/linux/reiserfs_fs.h
+++ b/include/linux/reiserfs_fs.h
@@ -1557,7 +1557,7 @@
 /* When inserting an item. */
 #define M_INSERT	'i'
 /* When inserting into (directories only) or appending onto an already
-   existant item. */
+   existent item. */
 #define M_PASTE		'p'
 /* When deleting an item. */
 #define M_DELETE	'd'