Correct a regression in the previous change, r322459. Specifically, the
removal of the "blk" parameter from blst_meta_alloc() had the unintended
effect of generating an out-of-range allocation when the cursor reaches
the end of the tree if the number of managed blocks in the tree equals
the so-called "radix" (which in the blist code is not the standard notion
of what a radix is but rather the maximum number of leaves in a tree of
the current height.) In other words, only certain swap configurations
were affected, which is why earlier testing did not reveal the problem.

Submitted by: Doug Moore <dougm@rice.edu>
Reported by: pho, kib
Tested by: pho
X-MFC with: r322459
Differential Revision: https://reviews.freebsd.org/D12106