Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
diff --git a/fs/nfs/callback_xdr.c b/fs/nfs/callback_xdr.c
index 14e0f93..00ecf62 100644
--- a/fs/nfs/callback_xdr.c
+++ b/fs/nfs/callback_xdr.c
@@ -241,7 +241,7 @@
 
 	args->cbl_layout_type = ntohl(*p++);
 	/* Depite the spec's xdr, iomode really belongs in the FILE switch,
-	 * as it is unuseable and ignored with the other types.
+	 * as it is unusable and ignored with the other types.
 	 */
 	iomode = ntohl(*p++);
 	args->cbl_layoutchanged = ntohl(*p++);