mon/OSDMonitor: make pgp_num track pg_num more consistently

mon/OSDMonitor: make pgp_num track pg_num more consistently

Previously we were automatically adjusting pgp_num_target on a
pg_num_target change *only* when decreasing pg_num.  Instead, make
pgp_num (continue to) track pg_num if it currently matches.  If it ever
is set differently than pg_num, leave it different (unless/until it
matches again).

This is still slightly weird, but I think in practice it is good enough.
In the rare case that the admin manually sets pgp_num to something
different than pg_num, they probably won't also be using automagic
pg_num adjustment that might make them match and start tracking again.

Signed-off-by: Sage Weil <sage@redhat.com>