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/page_cgroup.h b/include/linux/page_cgroup.h
index f5de21d..961ecc7 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -138,7 +138,7 @@
 
 #define PCG_ARRAYID_OFFSET	(BITS_PER_LONG - PCG_ARRAYID_WIDTH)
 /*
- * Zero the shift count for non-existant fields, to prevent compiler
+ * Zero the shift count for non-existent fields, to prevent compiler
  * warnings and ensure references are optimized away.
  */
 #define PCG_ARRAYID_SHIFT	(PCG_ARRAYID_OFFSET * (PCG_ARRAYID_WIDTH != 0))