vsprintf: Use non-atomic bitmap API when applicable

The 'set' bitmap is local to this function. No concurrent access to it is
possible.
So prefer the non-atomic '__[set|clear]_bit()' function to save a few
cycles.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/1abf81a5e509d372393bd22041eed4ebc07ef9f7.1638023178.git.christophe.jaillet@wanadoo.fr