export/fuse: Give SET_ATTR_SIZE its own branch

In order to support changing other attributes than the file size in
fuse_setattr(), we have to give each its own independent branch.  This
also applies to the only attribute we do support right now.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210625142317.271673-4-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/fuse.c

@@ -417,20 +417,22 @@ static void fuse_setattr(fuse_req_t req, fuse_ino_t inode, struct stat *statbuf,
     FuseExport *exp = fuse_req_userdata(req);
     int ret;
 
-    if (!exp->writable) {
-        fuse_reply_err(req, EACCES);
-        return;
-    }
-
     if (to_set & ~FUSE_SET_ATTR_SIZE) {
         fuse_reply_err(req, ENOTSUP);
         return;
     }
 
-    ret = fuse_do_truncate(exp, statbuf->st_size, true, PREALLOC_MODE_OFF);
-    if (ret < 0) {
-        fuse_reply_err(req, -ret);
-        return;
+    if (to_set & FUSE_SET_ATTR_SIZE) {
+        if (!exp->writable) {
+            fuse_reply_err(req, EACCES);
+            return;
+        }
+
+        ret = fuse_do_truncate(exp, statbuf->st_size, true, PREALLOC_MODE_OFF);
+        if (ret < 0) {
+            fuse_reply_err(req, -ret);
+            return;
+        }
     }
 
     fuse_getattr(req, inode, fi);