asm-generic: add a flush_icache_user_range stub

Define flush_icache_user_range to flush_icache_range unless the
architecture provides its own implementation.

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/20200515143646.3857579-21-hch@lst.de


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>