sys_swapon: fix inode locking

A conflict between 52c50567 ("mm: swap: unlock swapfile inode mutex
before closing file on bad swapfiles") and 83ef99be ("sys_swapon:
remove did_down variable") caused a double unlock of the inode mutex
(once in bad_swap: before the filp_close, once at the end just before
returning).

The patch which added the extra unlock cleared did_down to avoid
unlocking twice, but the other patch removed the did_down variable.

To fix, set inode to NULL after the first unlock, since it will be used
after that point only for the final unlock.

While checking this patch, I found a path which could unlock without
locking, in case the same inode was added as a swapfile twice. To fix,
move the setting of the inode variable further down, to just before
claim_swapfile, which will lock the inode before doing anything else.

Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Eric B Munson <emunson@mgebm.net>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>