Initial QSfera import

Server/vendor/github.com/ceph/go-ceph/cephfs/file_fd.go

@@ -0,0 +1,15 @@
+package cephfs
+
+// Fd returns the integer open file descriptor in cephfs.
+// NOTE: It doesn't make sense to consume the returned integer fd anywhere
+// outside CephFS and is recommended not to do so given the undefined behaviour.
+// Also, as seen with the Go standard library, the fd is only valid as long as
+// the corresponding File object is intact in the sense that an fd from a closed
+// File object is invalid.
+func (f *File) Fd() int {
+	if f == nil || f.mount == nil {
+		return -1
+	}
+
+	return int(f.fd)
+}