FreeBSD: Fix uninitialized variable error

On FreeBSD errno is defined as (* __error()), which means compiler
can't say whether two consecutive reads will return the same.
And without this knowledge the reported error is formally right.

Caching of the errno in local variable fixes the issue.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Rob Norris <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Closes #17975

Author: amotin

lib/libspl/tunables.c

@@ -124,10 +124,12 @@ zfs_tunable_parse_int(const char *val, intmax_t *np,
 {
 	intmax_t n;
 	char *end;
+	int err;
+
 	errno = 0;
 	n = strtoimax(val, &end, 0);
-	if (errno != 0)
-		return (errno);
+	if ((err = errno) != 0)
+		return (err);
 	if (*end != '\0')
 		return (EINVAL);
 	if (n < min || n > max)
@@ -142,10 +144,12 @@ zfs_tunable_parse_uint(const char *val, uintmax_t *np,
 {
 	uintmax_t n;
 	char *end;
+	int err;
+
 	errno = 0;
 	n = strtoumax(val, &end, 0);
-	if (errno != 0)
-		return (errno);
+	if ((err = errno) != 0)
+		return (err);
 	if (*end != '\0')
 		return (EINVAL);
 	if (strchr(val, '-'))