replace_fd() began with "eats a reference, tries to insert into
descriptor table" semantics; at some point I'd switched it to
much saner current behaviour ("try to insert into descriptor
table, grabbing a new reference if inserted; caller should do
fput() in any case"), but forgot to update the callers.
Mea culpa...

[Spotted by Pavel Roskin, who has really weird system with pipe-fed
coredumps as part of what he considers a normal boot ;-)]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>