Prior to this commit, the loader would perform readaheads of up
to 128 kB; when booting on a UFS filesystem this resulted in a
series of 160 kB reads (32 kB request + 128 kB readahead).
This commit allows readaheads to be longer, subject to a total
I/O size limit of 256 kB; i.e. 32 kB read requests will have added
readaheads of up to 224 kB.
In my testing on an EC2 c5.xlarge instance, this change reduces the
boot time by roughly 80 ms.