CVE-2023-52618: 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 "devsearchpath" can technically be as large as PATHMAX, there was a risk of truncation when copying it and a second string into "fullpath" since it was also PATHMAX sized. The W=1 builds were reporting this warning:

drivers/block/rnbd/rnbd-srv.c: In function 'processmsgopen.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(fullpath, PATHMAX, "%s/%s", | ^~ In function 'rnbdsrvgetfullpath', inlined from 'processmsgopen.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(fullpath, PATHMAX, "%s/%s", | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 617 | devsearchpath, devname); | ~~~~~~~~~~~~~~~~~~~~~~~~~~

To fix this, unconditionally check for truncation (as was already done for the case where "%SESSNAME%" was present).