Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 0d9a036..befc875 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -787,7 +787,7 @@
  * jiffies for either a BDI to exit congestion of the given @sync queue
  * or a write to complete.
  *
- * In the absense of zone congestion, cond_resched() is called to yield
+ * In the absence of zone congestion, cond_resched() is called to yield
  * the processor if necessary but otherwise does not sleep.
  *
  * The return value is 0 if the sleep is for the full timeout. Otherwise,