idr_replace() returns the old value on success, not 0.

Link: http://lkml.kernel.org/r/20170918162642.37511-1-ebiggers3@gmail.comSigned-off-by: NEric Biggers <ebiggers@google.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>