Avoid a dubious assignment to bio_data in aio_qbio().

A user pointer is not a suitable value for bio_data and the next block
of code always overwrites bio_data anyway. Just use cb->aio_buf
directly in the call to vm_fault_quick_hold_pages().

Reviewed by: kib
Obtained from: CheriBSD
MFC after: 1 month
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26595