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

I tested this by running vstart with two OSDs. Doing split on a pool while a different pool was creating succeeded; trying to split a still-creating pool did not.

gregf@kai:~/ceph/src [next]$ ./ceph osd pool create foo 100 && ./ceph osd pool set data pg_num 50
*** DEVELOPER MODE: setting PATH, PYTHONPATH and LD_LIBRARY_PATH ***
pool 'foo' created
*** DEVELOPER MODE: setting PATH, PYTHONPATH and LD_LIBRARY_PATH ***
set pool 0 pg_num to 50
gregf@kai:~/ceph/src [next]$ ./ceph osd pool set data pg_num 100
*** DEVELOPER MODE: setting PATH, PYTHONPATH and LD_LIBRARY_PATH ***
Error EAGAIN: currently creating pgs, wait