OSDMonitor: be a little nicer about letting users do pg splitting

We were previously blocking pg splits whenever pg creations were in-
progress, but we only really need to avoid splitting any pgs which are
currently being created. Let the user set a different pg_num if there
are no creating PGs on the pool in question.

Fixes: #6673, take two

Signed-off-by: Greg Farnum <greg@inktank.com>