block: fixup rq/bio dcache page flushing for ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE

block: fixup rq/bio dcache page flushing for ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE

On archs with ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE set, the conversion
on the flush functions was not complete:

   fs/bio.c: In function 'bio_flush_dcache_pages':
>> fs/bio.c:1693:2: error: incompatible types when assigning to type 'struct bio_vec *' from type 'struct bio_vec'
   fs/bio.c:1693:2: warning: left-hand operand of comma expression has no effect [-Wunused-value]
--
   block/blk-core.c: In function 'rq_flush_dcache_pages':
>> block/blk-core.c:2745:2: error: incompatible types when assigning to type 'struct bio_vec *' from type 'struct bio_vec'
   block/blk-core.c:2745:2: warning: left-hand operand of comma expression has no effect [-Wunused-value]

Fix them up.

Signed-off-by: Jens Axboe <axboe@kernel.dk>