asm-generic: fix the inclusion guards for cacheflush.h

cacheflush.h uses a somewhat to generic include guard name that clashes
with various arch files.  Use a more specific one.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: http://lkml.kernel.org/r/20200515143646.3857579-6-hch@lst.deSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/cacheflush.h

 /* SPDX-License-Identifier: GPL-2.0 */
-#ifndef __ASM_CACHEFLUSH_H
-#define __ASM_CACHEFLUSH_H
+#ifndef _ASM_GENERIC_CACHEFLUSH_H
+#define _ASM_GENERIC_CACHEFLUSH_H
 
 /* Keep includes the same across arches.  */
 #include <linux/mm.h>
@@ -109,4 +109,4 @@ static inline void flush_cache_vunmap(unsigned long start, unsigned long end)
 	memcpy(dst, src, len)
 #endif
 
-#endif /* __ASM_CACHEFLUSH_H */
+#endif /* _ASM_GENERIC_CACHEFLUSH_H */