PCI/MSI: Simplify populate_msi_sysfs()

Simplify populate_msi_sysfs() by

  - Swapping the order of the two allocations and storing the
    msi_dev_attr-derived pointer right after allocation, allowing the
    cleanup code to pick things up without extra effort.

  - Using kasprintf() instead of the kmalloc()/sprintf() pair.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>