mm/shmem: fix shmem_swapin() race with swapoff

mm/shmem: fix shmem_swapin() race with swapoff

When I was investigating the swap code, I found the below possible race
window:

CPU 1                                         CPU 2
-----                                         -----
shmem_swapin
  swap_cluster_readahead
    if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
                                              swapoff
                                                ..
                                                si->swap_file = NULL;
                                                ..
    struct inode *inode = si->swap_file->f_mapping->host;[oops!]

Close this race window by using get/put_swap_device() to guard against
concurrent swapoff.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or not")
Signed-off-by: Miaohe Lin <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>