filemap: minor cleanup for filemap_write_and_wait_range

Restructure the logic in filemap_write_and_wait_range to simplify the code
and make it more consistent with file_write_and_wait_range. No functional
change intended.

Link: https://lkml.kernel.org/r/20220627132351.55680-1-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>