zvol_id: make array length properly known at compile time

Using strlen() in an static array declaration is a GCC extension. Clang
calls it "gnu-folding-constant" and warns about it, which breaks the
build. If it were widespread we could just turn off the warning, but
since there's only one case, lets just change the array to an explicit
size.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17954

udev/zvol_id.c

@@ -67,7 +67,7 @@ main(int argc, const char *const *argv)
 		return (1);
 	}
 
-	char zvol_name[MAXNAMELEN + strlen("-part") + 10];
+	char zvol_name[MAXNAMELEN+15];
 	if (ioctl(fd, BLKZNAME, zvol_name) == -1) {
 		fprintf(stderr, "%s: BLKZNAME: %s\n",
 		    dev_name, strerror(errno));