Fix misspellings of "invocation" in comments.

Some comments misspell "invocation"; this fixes them. No code
changes.

Signed-off-by: Adam Buchbinder <adam.buchbinder@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/fs/buffer.c b/fs/buffer.c
index 6fa5302..1d920ba 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2893,7 +2893,7 @@
 
 	/*
 	 * The page straddles i_size.  It must be zeroed out on each and every
-	 * writepage invokation because it may be mmapped.  "A file is mapped
+	 * writepage invocation because it may be mmapped.  "A file is mapped
 	 * in multiples of the page size.  For a file that is not a multiple of
 	 * the  page size, the remaining memory is zeroed when mapped, and
 	 * writes to that region are not written out to the file."