mm/filemap: add helper for finding pages

mainline inclusion
from mainline-v5.12-rc1
commit c7bad633
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I3ZE5V
CVE: NA

-------------------------------------------------

There is a lot of common code in find_get_entries(),
find_get_pages_range() and find_get_pages_range_tag().  Factor out
find_get_entry() which simplifies all three functions.

[willy@infradead.org: remove VM_BUG_ON_PAGE()]
  Link: https://lkml.kernel.org/r/20201124041507.28996-2-willy@infradead.orgLink: https://lkml.kernel.org/r/20201112212641.27837-7-willy@infradead.org



Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Tong Tiangen <tongtiangen@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>