We're supposed to initialize the list head of an object before adding it
to another list.  Fix that, and stop using the kmem_{alloc,free} calls
from the Irix days.

Fixes: 174edb0e ("xfs: store in-progress CoW allocations in the refcount btree")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>