os/bluestore: fix assertion in StupidAllocator::get_fragmentation

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.

Fixes: https://tracker.ceph.com/issues/43297
Note: This was a clean cherry-pick from master, but p2roundup was
introduced since mimic release, use P2ROUNDUP instead

Signed-off-by: Igor Fedotov <ifedotov@suse.com>
(cherry picked from commit a60b2316ce0bed28c468043cff4cab5e61b1a694)
Signed-off-by: Lei Liu <liulei3@360.cn>