bio_split — split a bio
| struct bio * bio_split ( | struct bio * bio, | 
| int sectors, | |
| gfp_t gfp, | |
| struct bio_set * bs ); | 
biobio to split
sectors
     number of sectors to split from the front of bio
    
gfpgfp mask
bsbio set to allocate from
   Allocates and returns a new bio which represents sectors from the start of
   bio, and updates bio to represent the remaining sectors.
   
   Unless this is a discard request the newly allocated bio will point
   to bio's bi_io_vec; it is the caller's responsibility to ensure that
   bio is not freed before the split.