OSDMonitor: prevent extreme multipliers on PG splits

Fixes: #6922
Backport: emperor

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit f57dad6461171c903e8b5255eaed300374b00e74)

Conflicts:

	src/mon/OSDMonitor.cc
OSDMonitor: use a different approach to prevent extreme multipliers on PG splits

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit d8ccd73968fbd0753ca08916ebf1062cdb4d5ac1)

Conflicts:

	src/mon/OSDMonitor.cc
Merge pull request #1316 from ceph/dumpling-6922

Dumpling: Prevent extreme PG split multipliers

Reviewed-by: Samuel Just <sam.just@inktank.com>