once: rename _SLOW to _SLEEPABLE

The _SLOW designation wasn't really descriptive of anything. This is
meant to be called from process context when it's possible to sleep. So
name this more aptly _SLEEPABLE, which better fits its intended use.

Fixes: 62c07983

 ("once: add DO_ONCE_SLOW() for sleepable contexts")
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20221003181413.1221968-1-Jason@zx2c4.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>