uml: Eliminate temporary buffer in eth_configure

Avoid using the temporary buffer introduced by previous patch to hold the
device name.

Btw, avoid leaking device on an error path.  Other error paths may need
cleanup.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>