blocklevel: smart_write: Avoid indirectly testing formal parameters
blocklevel: smart_write: Avoid indirectly testing formal parameters

The early-exit tests write_buf, but write_buf is assigned to buf on
declaration. Test buf directly instead to avoid unnecessary indirection.

Signed-off-by: Andrew Jeffery <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@aj.id.au>
Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

open-power/skiboot,qemu/skiboot,legoater/skiboot,open-power/skiboot,open-power/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,qemu/skiboot,shenki/skiboot,shenki/skiboot,open-power/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,legoater/skiboot