ext3: Don't warn from writepage when readonly inode is spotted after error

commit 33c104d415e92a51aaf638dc3d93920cfa601e5c upstream.

WARN_ON_ONCE(IS_RDONLY(inode)) tends to trip when filesystem hits error and is
remounted read-only. This unnecessarily scares users (well, they should be
scared because of filesystem error, but the stack trace distracts them from the
right source of their fear ;-). We could as well just remove the WARN_ON but
it's not hard to fix it to not trip on filesystem with errors and not use more
cycles in the common case so that's what we do.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>