mqueuefs: fix statfs report to not signal file system full

Synthetic file systems that do not actually allocate file system
blocks or inodes should report that they have space available and
that they provide 0 inodes, in order to prevent capacity monitoring
tools from warning about resource exhaustion.

This has been fixed in all other synthetic file systems in base in
commit 88a795e80c0, but this file was overlooked since its name does
not indicate that it also provides a file system.

MFC after: 1 month