mm/gup.c: Fix return value for __gup_longterm_locked()

mm/gup.c: Fix return value for __gup_longterm_locked()

__get_user_pages_locked() may return the number of pages less than
nr_pages.  So __gup_longterm_locked() have to return the number of pages
__get_user_pages_locked() returns if it succeeded, not nr_pages requested.

Link: https://lkml.kernel.org/r/20220821183547.950370-1-syoshida@redhat.com
Fixes: 61c63c2076d9 (mm/gup.c: simplify and fix check_and_migrate_movable_pages() return codes)
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reported-by: syzbot+616ff0452fec30f4dcfd@syzkaller.appspotmail.com
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>