__add_to_swap_cache unconditionally sets the page locked, which can be a bit
alarming to the unsuspecting reader: in the code paths where the page is
visible to other CPUs, the page should be (and is) already locked.

Instead, just add a check to ensure the page is locked here, and teach the one
path relying on the old behaviour to call SetPageLocked itself.

[hugh@veritas.com: locking fix]
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NHugh Dickins <hugh@veritas.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>