Move the redirection to stderr out of the cmd variable assignment

Putting 2>/dev/null in cmd= escapes the redirection operation, which causes
mdconfig to think it's a filename

MFC after: 2 weeks
X-MFC with: r273627
Sponsored by: EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273628 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f