file_ram_alloc: unify mem-path,mem-prealloc error handling

-mem-prealloc asks to preallocate memory residing on -mem-path path.

Currently QEMU exits in case:

- Memory file has been created but allocation via explicit write
fails.

And it fallbacks to malloc in case:
- Querying huge page size fails.
- Lack of sync MMU support.
- Open fails.
- mmap fails.

Have the same behaviour for all cases: fail in case -mem-path and
-mem-prealloc are specified for regions where the requested size is
suitable for hugepages.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1031,7 +1031,7 @@ static void *file_ram_alloc(RAMBlock *block,
 
     hpagesize = gethugepagesize(path);
     if (!hpagesize) {
-        return NULL;
+        goto error;
     }
 
     if (memory < hpagesize) {
@@ -1040,7 +1040,7 @@ static void *file_ram_alloc(RAMBlock *block,
 
     if (kvm_enabled() && !kvm_has_sync_mmu()) {
         fprintf(stderr, "host lacks kvm mmu notifiers, -mem-path unsupported\n");
-        return NULL;
+        goto error;
     }
 
     /* Make name safe to use with mkstemp by replacing '/' with '_'. */
@@ -1058,7 +1058,7 @@ static void *file_ram_alloc(RAMBlock *block,
     if (fd < 0) {
         perror("unable to create backing store for hugepages");
         g_free(filename);
-        return NULL;
+        goto error;
     }
     unlink(filename);
     g_free(filename);
@@ -1078,7 +1078,7 @@ static void *file_ram_alloc(RAMBlock *block,
     if (area == MAP_FAILED) {
         perror("file_ram_alloc: can't mmap RAM pages");
         close(fd);
-        return (NULL);
+        goto error;
     }
 
     if (mem_prealloc) {
@@ -1122,6 +1122,12 @@ static void *file_ram_alloc(RAMBlock *block,
 
     block->fd = fd;
     return area;
+
+error:
+    if (mem_prealloc) {
+        exit(1);
+    }
+    return NULL;
 }
 #else
 static void *file_ram_alloc(RAMBlock *block,