block: optimise bvec_iter_advance()

bvec_iter_advance() is quite popular, but compilers fail to do proper
alias analysis and optimise it good enough. The assembly is checked
for gcc 9.2, x86-64.

- remove @iter->bi_size from min(...), as it's always less than @bytes.
Modify at the beginning and forget about it.

- the compiler isn't able to collapse memory dependencies and remove
writes in the loop. Help it by explicitely using local vars.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>