libata: bump ->qc_active to a 64-bit type

This is in preparation for allowing full usage of the tag space,
which means that our reserved error handling command will be
using an internal tag value of 32. This doesn't fit in a u32, so
move to a u64.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Tejun Heo <tj@kernel.org>