block/rnbd-srv: Check for unlikely string overflow

In the Linux kernel, the following vulnerability has been resolved:

block/rnbd-srv: Check for unlikely string overflow

Since "dev_search_path" can technically be as large as PATH_MAX,

there was a risk of truncation when copying it and a second string

into "full_path" since it was also PATH_MAX sized. The W=1 builds were

reporting this warning:

drivers/block/rnbd/rnbd-srv.c: In function 'process_msg_open.isra':

drivers/block/rnbd/rnbd-srv.c:616:51: warning: '%s' directive output may be truncated writing up to 254 bytes into a region of size between 0 and 4095 [-Wformat-truncation=]

616 | snprintf(full_path, PATH_MAX, "%s/%s",

| ^~

In function 'rnbd_srv_get_full_path',

inlined from 'process_msg_open.isra' at drivers/block/rnbd/rnbd-srv.c:721:14: drivers/block/rnbd/rnbd-srv.c:616:17: note: 'snprintf' output between 2 and 4351 bytes into a destination of size 4096

616 | snprintf(full_path, PATH_MAX, "%s/%s",

| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

617 | dev_search_path, dev_name);

| ~~~~~~~~~~~~~~~~~~~~~~~~~~

To fix this, unconditionally check for truncation (as was already done

for the case where "%SESSNAME%" was present).