MFC 305722

MFC 305722

    x86: Use sx lock for interrupt sources.

    - Certain pic_assign_cpu, e.g. msi_assign_cpu can have quite a long
      call chain.  For msi_assign_cpu, mutex makes complex PCI bridge
      drivers more tricky, e.g. sleep can note be called, etc, it will
      be pretty tricky for upcoming Hyper-V PCI bridge driver for PCI
      pass-through.
    - It is not used on any hot code path nor non-sleepable context, so
      sx should have the same effect as mutex.

    PIC list is still protected by mutex to keep suspend/resume work.

    Discussed with: jhb
    Reviewed by:    jhb
    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7784

git-svn-id: svn://svn.freebsd.org/base/stable/10@307244 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f