Add statx musl support
This MR adds musl support, adding the missing bit to make statx() available for the musl compilation target.
And removes the unused stat64() function. Currently, MountFds::get() uses statx() without checking its availability. But, even though PassthroughFs::new() checks if statx() is available, stat64() always returns a mount ID 0. This silently breaks '--announce-submounts', and with '--inode-file-handles' virtiofsd fails with a cryptic error, during mount.
Closes #27 (closed)
Edited by German Maglione