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/mm.h b/include/linux/mm.h
index 7606d7d..692dbae 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -608,7 +608,7 @@
 #endif
 
 /*
- * Define the bit shifts to access each section.  For non-existant
+ * Define the bit shifts to access each section.  For non-existent
  * sections we define the shift as 0; that plus a 0 mask ensures
  * the compiler will optimise away reference to them.
  */