tools/process_locker: Decrement writer count in drop handler

of ProcessLockSharedGuard.

We use a counter to determine if we can unlock the file again, but
we never actually decremented the writer count, so we held the
lock forever.

This fixes the issue that we could not start a garbage collect after
a reload, as long as the old process is still running, even when that
process has no active backup anymore but another long running task
(e.g. file download, terminal, etc.).

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/tools/process_locker.rs

@@ -55,7 +55,9 @@ impl  Drop for ProcessLockSharedGuard {
             if let Err(err) = nix::fcntl::fcntl(data.file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLKW(&op)) {
                 panic!("unable to drop writer lock - {}", err);
             }
-            data.writers = 0;
+        }
+        if data.writers > 0 {
+            data.writers -= 1;
         }
     }
 }