[IPoIB] Rename ipoib_create_qp() -> ipoib_init_qp() and fix error cleanup

ipoib_create_qp() no longer creates IPoIB's QP, so it shouldn't
destroy the QP on failure -- that unwinding happens elsewhere, so the
current code can cause a double free.  While we're at it, the
function's name should match what it actually does, so rename it to
ipoib_init_qp().

Signed-off-by: Roland Dreier <rolandd@cisco.com>