mpage: convert do_mpage_readpage() to return void type

mpage: convert do_mpage_readpage() to return void type

The return value of do_mpage_readpage() is arg->bio, which is already set
in the arg structure.  Returning it again is redundant.

This patch changes the return type to void since the caller doesn't care
about the return value.

Link: https://lkml.kernel.org/r/20250829023659.688649-2-chizhiling@163.com
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Sungjong Seo <sj1557.seo@samsung.com>
Cc: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>