fbdev/fb.h: silence warning with -Wsign-compare

Silence the warning when building with -Wsign-compare when fb.h is
included:

include/linux/fb.h: In function ‘__fb_pad_aligned_buffer’:
include/linux/fb.h:650:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (j = 0; j < s_pitch; j++)
                 ^
Signed-off-by: Brian W Hart <hartb@linux.vnet.ibm.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

include/linux/fb.h

@@ -642,7 +642,7 @@ static inline void unlock_fb_info(struct fb_info *info)
 static inline void __fb_pad_aligned_buffer(u8 *dst, u32 d_pitch,
 					   u8 *src, u32 s_pitch, u32 height)
 {
-	int i, j;
+	u32 i, j;
 
 	d_pitch -= s_pitch;