x86/irq_remapping: Sanitize posted_msi_supported()

x86/irq_remapping: Sanitize posted_msi_supported()

posted_msi_supported() is a misnomer as it actually checks whether it is
enabled or not. Aside of that this does not take CONFIG_X86_POSTED_MSI into
account which is required to actually use it.

Rename it to posted_msi_enabled() and make the return value depend on
CONFIG_X86_POSTED_MSI, which allows the compiler to eliminate the related
dead code and data if disabled:

  text	   data	    bss	    dec	    hex	filename
  10046	    701	   3296	  14043	   36db	drivers/iommu/intel/irq_remapping.o
   9904	    413	   3296	  13613	   352d	drivers/iommu/intel/irq_remapping.o

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251125214631.170499997@linutronix.de