WARN_ONCE(): use bool for boolean flag

Commit 70867453

 ("printk_once(): use bool
for boolean flag") changed printk_once() to use bool instead of int for
its guard variable.  Do the same change to WARN_ONCE() and WARN_ON_ONCE(),
for the same reasons.

This resulted in a reduction of 1462 bytes on a x86-64 defconfig:

   text    data     bss     dec     hex filename
8101271 1207116  992764 10301151         9d2edf vmlinux.before
8100553 1207148  991988 10299689         9d2929 vmlinux.after

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Roland Dreier <rolandd@cisco.com>
Cc: Daniel Walker <dwalker@fifo99.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>