Let the process die instead of panic on failure to allocate page tables

We panic all over the place on OOM, but it'd be good to slowly work towards slightly saner error handling.

Let the process die instead of panic on failure to allocate page tables
a2fb0954 · David Benjamin · 728f8126 · a2fb0954
--- a/kernel/vm.cc
+++ b/kernel/vm.cc
@@ -479,8 +479,10 @@ vmap::pagefault(uptr va, u32 err)
    return -1;
  atomic<pme_t> *pte = walkpgdir(pml4, va, 1);
-  if (pte == nullptr)
+  if (pte == nullptr) {
-    panic("pagefault: couldn't allocate page table");
+    cprintf("pagefault: couldn't allocate page table\n");
+    return -1;
+  }
 retry:
  pme_t ptev = pte->load();