genirq/msi: Simplify sysfs handling

The sysfs handling for MSI is a convoluted maze and it is in the way of
supporting dynamic expansion of the MSI-X vectors because it only supports
a one off bulk population/free of the sysfs entries.

Change it to do:

   1) Creating an empty sysfs attribute group when msi_device_data is
      allocated

   2) Populate the entries when the MSI descriptor is initialized

   3) Free the entries when a MSI descriptor is detached from a Linux
      interrupt.

   4) Provide functions for the legacy non-irqdomain fallback code to
      do a bulk population/free. This code won't support dynamic
      expansion.

This makes the code simpler and reduces the number of allocations as the
empty attribute group can be shared.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211206210749.224917330@linutronix.de