Embed bio in mp_bh rather than separate allocation.

multipath currently allocates an mp_bh and a bio for each
request.  With this patch, the bio is made to be part of the
mp_bh so there is only one allocation, and it from a private
pool (the bio was allocated from a shared pool).

Also remove "remaining" and "cmd" from mp_bh which aren't used.
And remove spare (unused) from multipath_private_data.