fs: Hold format.json readLock ref to avoid GC. (#4532)

Looks like if we follow pattern such as

```
_ = rlk
```

Go can potentially kick in GC and close the fd when
the reference is lost, only speculation is that
the cause here is `SetFinalizer` which is set on
`os.close()` internally in `os` stdlib.

This is unexpected and unsual endeavour for Go, but
we have to make sure the reference is never lost
and always dies with the server.

Fixes #4530

pkg/lock/lock_solaris.go

@@ -58,7 +58,7 @@ func lockedOpenFile(path string, flag int, perm os.FileMode, rlockType int) (*Lo
 	if err = syscall.FcntlFlock(f.Fd(), rlockType, &lock); err != nil {
 		f.Close()
 		if err == syscall.EAGAIN {
-			err = ErrLocked
+			err = ErrAlreadyLocked
 		}
 		return nil, err
 	}