lib/vsprintf.c: replace while with do-while in skip_atoi

All callers of skip_atoi have already checked for the first character
being a digit. In this case, gcc generates simpler code for a do
while-loop.

Signed-off-by: Rasmus Villemoes <[e-mail]>
Cc: Jiri Kosina <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/vsprintf.c

···
 {
 	int i = 0;
 
-	while (isdigit(**s))
+	do {
 		i = i*10 + *((*s)++) - '0';
+	} while (isdigit(**s));
 
 	return i;
 }