Fix oddity where mdadm did not recognise a relative path

Fix oddity where mdadm did not recognise a relative path

mdadm assumed that a pathname started with a "/", while an array
name didn't. This alters the logic so that if the first character
is not a "/" it tries to open an array, and if that fails it drops
through to the pathname code rather than terminating immediately
with an error.

Signed-off-by: Wol <anthony@youngman.org.uk>
Signed-off-by: Jes Sorensen <Jes.Sorensen@gmail.com>