Add sbuf variants ata_cmd_sbuf() and ata_res_sbuf(), and reimplement the
_string variants on top of this. This requires a change to the function
signature of ata_res_sbuf(). Its use in the tree seems to be very limited,
and the change makes it more consistent with the rest of the API.

Reviewed by: imp, mav, kenm
Sponsored by: Netflix
Differential Revision: D5940