reed_solomon: Fix kernel-doc

The current doc build warns:
./lib/reed_solomon/reed_solomon.c:287: WARNING: Unknown target name: "gfp".

This is because it misinterprets the "GFP_" that is part of the
description.  Change the description to avoid the problem.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>