genirq/msi: Populate sysfs entry only once

The MSI entries for multi-MSI are populated en bloc for the MSI descriptor,
but the current code invokes the population inside the per interrupt loop
which triggers a warning in the sysfs code and causes the interrupt
allocation to fail.

Move it outside of the loop so it works correctly for single and multi-MSI.

Fixes: bf5e758f

 ("genirq/msi: Simplify sysfs handling")
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/87leznqx2a.ffs@tglx