debian/patches: patch the value of the preferred dbroot

Since the saveconfig.json configuration file is saved in
/etc/rtslib-fb-target, the preferred dbroot location where the kernel
may store persistent data across reboots (PR or ALUA) has to be
updated too.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>