libct/specconv: improve checkPropertyName

Commit 029b73c1b replaced a regular expression with code checking the
characters. Despite what the comment said about ASCII, the check was
performed rune by rune, not byte by byte.

Note the check was still correct, basically comparing int32's, but the
byte by byte way is a tad faster and more straightforward. The change
also fixes the issue of a misleading comment.

Benchmark before/after shows a modest improvement:

name                 old time/op    new time/op    delta
CheckPropertyName-4     164ns ± 2%     123ns ± 2%  -24.73%  (p=0.029 n=4+4)

name                 old alloc/op   new alloc/op   delta
CheckPropertyName-4     96.0B ± 0%     64.0B ± 0%  -33.33%  (p=0.029 n=4+4)

name                 old allocs/op  new allocs/op  delta
CheckPropertyName-4      6.00 ± 0%      4.00 ± 0%  -33.33%  (p=0.029 n=4+4)

Fixes: 029b73c1b
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/specconv/spec_linux.go

@@ -538,8 +538,10 @@ func checkPropertyName(s string) error {
 	if len(s) < 3 {
 		return errors.New("too short")
 	}
-	// Check ASCII characters rather than Unicode runes.
-	for _, ch := range s {
+	// Check ASCII characters rather than Unicode runes,
+	// so we have to use indexes rather than range.
+	for i := 0; i < len(s); i++ {
+		ch := s[i]
 		if (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z') {
 			continue
 		}