fix(fuse,scripts): log why a create finds no parent and fit the metadata floor to FUSE

A create that answered ENOENT left no trace in the log. It now warns
when the parent folder is no longer known, and when the parent left the
tree while its children were being listed, naming the parent inode and
the new name.

The acceptance script's metadata throughput floor was 20 operations per
second, which a FUSE mount cannot reach: each create and unlink waits for
Drive before it returns, about 350 ms apiece. On a FUSE mount the floor
is now 3 operations per second; other paths keep 20. The mount type is
read from /proc/self/mountinfo.

crates/pdfs-fuse/src/filesystem.rs

@@ -1464,6 +1464,12 @@ impl ProtonFs {
             return;
         }
         if let Err(e) = self.core.ensure_children(parent) {
+            // The parent was looked up a moment ago, so ENOENT here means it
+            // left the tree in between. Log it: the caller only sees "no such
+            // file" on a create.
+            if e == Errno::ENOENT {
+                warn!(parent, name, "create: the parent folder is no longer known");
+            }
             reply.error(e);
             return;
         }
@@ -1472,6 +1478,10 @@ impl ProtonFs {
             match st.entries.get(&parent) {
                 Some(e) => e.uid.clone(),
                 None => {
+                    warn!(
+                        parent,
+                        name, "create: the parent folder left the tree during its listing"
+                    );
                     reply.error(Errno::ENOENT);
                     return;
                 }