BUG/MINOR: shctx: do not look for available blocks when the first one is enough

BUG/MINOR: shctx: do not look for available blocks when the first one is enough

In shctx_row_reserve_hot() we only leave if we've found the exact
requested size instead of at least as large, as is documented. This
results in extra lookups and free calls in the avail loop while it is
not needed, and participates to seeing a negative data_len early as
spotted in previous bugs.

It doesn't seem to have any other impact however, but it's better to
backport it to stable branches.