mm: madvise: fix MADV_WILLNEED on shmem swapouts

mm: madvise: fix MADV_WILLNEED on shmem swapouts

MADV_WILLNEED currently does not read swapped out shmem pages back in.

0cd6144aadd2 ("mm + fs: prepare for non-page entries in page cache radix
trees") made find_get_page() filter exceptional radix tree entries but
failed to convert all find_get_page() callers that WANT exceptional
entries over to find_get_entry().  One of them is shmem swap readahead in
madvise, which now skips over any swap-out records.

Convert it to find_get_entry().

Fixes: 0cd6144aadd2 ("mm + fs: prepare for non-page entries in page cache radix trees")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>