xen/intr: allow xen_intr_release_isrc() to reuse lock

[ehem_freebsd_m5p.com]

Summary

Calling mtx_unlock() only to immediately call mtx_lock() isn't so great.
Since xen_intr_unbind() already has the main lock held before calling
xen_intr_release_isrc(), avoid the back to back unlock then lock.

[ehem_freebsd_m5p.com]

Kind of a "Hey, look what D32876 allows us to do!"

[ehem_freebsd_m5p.com]

Yes, this got into Phabricator before final testing. Indeed, it is broken. The fix is pretty obvious.

Upon examination I suspect things may head in a different direction.

[ehem_freebsd_m5p.com]

This isn't even the slightest bit of a priority. Worse, this does indeed make locking harrier.