Avoid computing strlen() inside loops

Compiling with -O0 (no proper optimizations), strlen() call
in loops for comparing the size, isn't being called/initialized
before the actual loop gets started, which causes n-numbers of
strlen() calls (as long as the string is). Keeping the length
before entering in the loop is a good idea.

On some places, even with -O2, both GCC and Clang can't
recognize this pattern, which seem to happen in an array
of char pointer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: rilysh <nightquick@proton.me>
Closes #16584

udev/zvol_id.c

@@ -56,6 +56,7 @@ main(int argc, const char *const *argv)
 		return (1);
 	}
 	const char *dev_name = argv[1];
+	size_t i, len;
 
 	int fd;
 	struct stat sb;
@@ -73,11 +74,13 @@ main(int argc, const char *const *argv)
 	}
 
 	const char *dev_part = strrchr(dev_name, 'p');
+	len = strlen(zvol_name);
 	if (dev_part != NULL) {
-		sprintf(zvol_name + strlen(zvol_name), "-part%s", dev_part + 1);
+		sprintf(zvol_name + len, "-part%s", dev_part + 1);
+		len = strlen(zvol_name);
 	}
 
-	for (size_t i = 0; i < strlen(zvol_name); ++i)
+	for (i = 0; i < len; ++i)
 		if (isblank(zvol_name[i]))
 			zvol_name[i] = '+';