ocfs2: Use ERR_CAST() to avoid cross-structure cast

When trying to propagate an error result, the error return path attempts
to retain the error, but does this with an open cast across very different
types, which the upcoming structure layout randomization plugin flags as
being potentially dangerous in the face of randomization. This is a false
positive, but what this code actually wants to do is use ERR_CAST() to
retain the error value.

Cc: Mark Fasheh <mfasheh@versity.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Kees Cook <keescook@chromium.org>