mm: convert find_get_entry to return the head page

mm: convert find_get_entry to return the head page

There are only four callers remaining of find_get_entry().
get_shadow_from_swap_cache() only wants to see shadow entries and doesn't
care about which page is returned.  Push the find_subpage() call into
find_lock_entry(), find_get_incore_page() and pagecache_get_page().

[[email protected]: fix oops]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: William Kucharski <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>