bio_alloc_bioset — allocate a bio for I/O
the GFP_ mask given to the slab allocator
number of iovecs to pre-allocate
the bio_set to allocate from
bio_alloc_bioset will first try it's on mempool to satisfy the allocation. If __GFP_WAIT is set then we will block on the internal pool waiting for a struct bio to become free.
allocate bio and iovecs from the memory pools specified by the bio_set structure.