powerpc/radix: take the pmap lock in mmu_radix_sync_icache()

mmu_radix_sync_icache() walked the page tables with an unlocked
pmap_extract() and passed the result straight to PHYS_TO_DMAP(),
checking only that it was non-zero.  Nothing keeps the mapping - or the
page table page holding it - alive across that window: if another thread
of the same process tears a mapping down concurrently, the page table
page can be freed and reused, so pmap_extract() reads arbitrary memory
and returns a bogus physical address.  __syncicache() then dereferences
an unmapped direct map address and the kernel takes a data storage
interrupt:

  fatal kernel trap:
     exception       = 0x300 (data storage interrupt)
     virtual address = 0xc003317ca6022a00
     dsisr           = 0x40000000
     srr0            = 0xc000000000f59460 (__syncicache)
     lr              = 0xc000000000f23588 (mmu_radix_sync_icache)
            pid = 23878, comm = skyframe-evaluator-
  panic: data storage interrupt trap

The faulting addresses decode to physical addresses far beyond installed
memory (~140 TB and ~900 TB on a 256 GB machine), i.e. translations that
never existed.

The hash MMU implementation of the same method, moea64_sync_icache(),
already holds PMAP_LOCK() across the loop; do the same here.
mmu_radix_extract() does not acquire the pmap lock itself, so this
introduces no recursion.

JIT workloads reach this path constantly: ppc_instr_emulate() calls
pmap_sync_icache() on the faulting address for the SIGILL "second
chance" retry, so a multithreaded JVM executing freshly written code
races against its own threads' mmap/munmap.  Every panic observed here
was in a JVM thread.

Tested on POWER9 (radix MMU) with a bazel/JVM build loop that previously
panicked the machine twice within ten minutes: afterwards 13 consecutive
builds and more than 10 hours of uptime with no panic, on both
15.1-RELEASE and 16.0-CURRENT.

MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D59311
Reviewed by:	jhibbits, adrian

sys/powerpc/aim/mmu_radix.c

@@ -5978,6 +5978,7 @@ mmu_radix_sync_icache(pmap_t pm, vm_offset_t va, vm_size_t sz)
 	if (__predict_false(pm == NULL))
 		pm = &curthread->td_proc->p_vmspace->vm_pmap;
 
+	PMAP_LOCK(pm);
 	while (sz > 0) {
 		pa = pmap_extract(pm, va);
 		sync_sz = PAGE_SIZE - (va & PAGE_MASK);
@@ -5989,6 +5990,7 @@ mmu_radix_sync_icache(pmap_t pm, vm_offset_t va, vm_size_t sz)
 		va += sync_sz;
 		sz -= sync_sz;
 	}
+	PMAP_UNLOCK(pm);
 }
 
 static __inline void