os/bluestore: fix assertion in StupidAllocator::get_fragmentation

One might face an assertion (assert(intervals <= max_intervals))
in StupidAllocator::get_fragmentation method for clusters created
by early Luminous releases and before. The root cause is that block
volume size wasn't aligned with min_alloc_size and hence we missed
that last fraction interval during max_interval calculation.

Signed-off-by: Igor Fedotov <ifedotov@suse.com>
(cherry picked from commit a60b231)

src/os/bluestore/StupidAllocator.cc

@@ -281,7 +281,7 @@ double StupidAllocator::get_fragmentation(uint64_t alloc_unit)
   uint64_t intervals = 0;
   {
     std::lock_guard<std::mutex> l(lock);
-    max_intervals = num_free / alloc_unit;
+    max_intervals = p2roundup(num_free, alloc_unit) / alloc_unit;
     for (unsigned bin = 0; bin < free.size(); ++bin) {
       intervals += free[bin].num_intervals();
     }