Commit 
70867453092297be9afb2249e712a1f960ec0a09 ("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>
 
 #endif
 
 #define WARN_ON_ONCE(condition)        ({                              \
-       static int __warned;                                    \
+       static bool __warned;                                   \
        int __ret_warn_once = !!(condition);                    \
                                                                \
        if (unlikely(__ret_warn_once))                          \
                if (WARN_ON(!__warned))                         \
-                       __warned = 1;                           \
+                       __warned = true;                        \
        unlikely(__ret_warn_once);                              \
 })
 
 #define WARN_ONCE(condition, format...)        ({                      \
-       static int __warned;                                    \
+       static bool __warned;                                   \
        int __ret_warn_once = !!(condition);                    \
                                                                \
        if (unlikely(__ret_warn_once))                          \
                if (WARN(!__warned, format))                    \
-                       __warned = 1;                           \
+                       __warned = true;                        \
        unlikely(__ret_warn_once);                              \
 })