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

cmd/ztest.c

@@ -685,15 +685,17 @@ static int
 str2shift(const char *buf)
 {
 	const char *ends = "BKMGTPEZ";
-	int i;
+	int i, len;
 
 	if (buf[0] == '\0')
 		return (0);
-	for (i = 0; i < strlen(ends); i++) {
+
+	len = strlen(ends);
+	for (i = 0; i < len; i++) {
 		if (toupper(buf[0]) == ends[i])
 			break;
 	}
-	if (i == strlen(ends)) {
+	if (i == len) {
 		(void) fprintf(stderr, "ztest: invalid bytes suffix: %s\n",
 		    buf);
 		usage(B_FALSE);