don't unlink an active swapfile

don't unlink an active swapfile

Peter Cordes is sorry that he rm'ed his swapfiles while they were in use,
he then had no pathname to swapoff.  It's a curious little oversight, but
not one worth a lot of hackery.  Kudos to Willy Tarreau for turning this
around from a discussion of synthetic pathnames to how to prevent unlink.
Mimic immutable: prohibit unlinking an active swapfile in may_delete()
(and don't worry my little head over the tiny race window).

Signed-off-by: Hugh Dickins <[email protected]>
Cc: Willy Tarreau <[email protected]>
Acked-by: Christoph Hellwig <[email protected]>
Cc: Peter Cordes <[email protected]>
Cc: Bodo Eggert <[email protected]>
Cc: David Newall <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>