BUG/MEDIUM: shctx: leave the block allocator when enough blocks are found

BUG/MEDIUM: shctx: leave the block allocator when enough blocks are found

In shctx_row_reserve_hot(), a missing break allows the avail loop to
loop for a while after having allocated the required blocks, possibly
leading to the point where it could trigger the watchdog after checking
up to 2 million blocks. In addition, the extra iteration may leave one
block assigned with size zero at the head of the avail list, and mark
it as being an isolated chain of 1 block. It's unclear whether this
could have had other consequences.

There is a non-negligible chance that it addreses bugs #1451 and #1284,
as the pattern observed in the loop looks exactly the same as the one
reported there in the crashes.

It's only marked medium because it is extremely hard to trigger. Here
the conditions were reproduced when starting 4k connections at once
requesting objects of random sizes between 0 and 20k to store them into
a small 1MB cache. However the watchdog will never trigger in such a case
so one needs to instrument the functions.

Thanks to Sohaib Ahmad and @g0uZ for providing useful traces.

This will need to be backported to all stable branches.