block: remove the bi_phys_segments field in struct bio

We only need the number of segments in the blk-mq submission path.
Remove the field from struct bio, and return it from a variant of
blk_queue_split instead of that it can passed as an argument to
those functions that need the value.

This also means we stop recounting segments except for cloning
and partial segments.

To keep the number of arguments in this how path down remove
pointless struct request_queue arguments from any of the functions
that had it and grew a nr_segs argument.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/biodoc.txt

@@ -436,7 +436,6 @@ struct bio {
        struct bvec_iter	bi_iter;	/* current index into bio_vec array */
 
        unsigned int	bi_size;     /* total size in bytes */
-       unsigned short 	bi_phys_segments; /* segments after physaddr coalesce*/
        unsigned short	bi_hw_segments; /* segments after DMA remapping */
        unsigned int	bi_max;	     /* max bio_vecs we can hold
                                         used as index into pool */