Avoid unneeded malloc/memcpy/free if there is no metadata on disk.

Submitted by:	Dmitry Luhtionov <dmitryluhtionov@gmail.com>
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275503 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f