Description of problem:

pg_num_check() should take into account the crush rule of the pool.

2153654 reported an error with setting the pool size to 1 using a specified crush rules. In the interest of the dev freeze timelines, the PR where the regression was introduced had been reverted.
Since the PR which made pg_num_check() to work accurately was reverted, we need to fix pg_num_check() to work accurately again.

How reproducible:
Always


Steps to Reproduce:
1. Using a cluster with 6 OSDs, set up a new crush rule where only 1 OSDs is added as root. (See "Additional info")

2. Assuming `mon_max_pg_per_osd` configuration value is 250, create a pool with 256 pg/pgp num with the new crush rule mentioned above.

Actual results:
Although we exceed the `mon_max_pg_per_osd` limit - the pool will be created successfully. 

Since the crush rule is not taken into account while checking, we divide the projected pg num by *all* of the OSDs in the cluster, we should divide by the root OSDs of the crush rule only.

```
pool 'pool_test ' created
```
Notice osd.0:
```
ID  CLASS  WEIGHT   REWEIGHT  SIZE     RAW USE  DATA     OMAP  META     AVAIL    %USE  VAR   PGS  STATUS
 0    ssd  0.09859   1.00000  101 GiB  1.0 GiB  780 KiB   0 B   23 MiB  100 GiB  0.99  1.00  257      up
 1    ssd  0.09859   1.00000  101 GiB  1.0 GiB  812 KiB   0 B   23 MiB  100 GiB  0.99  1.00    3      up
 2    ssd  0.09859   1.00000  101 GiB  1.0 GiB  780 KiB   0 B   19 MiB  100 GiB  0.99  1.00    2      up
 3    ssd  0.09859   1.00000  101 GiB  1.0 GiB  360 KiB   0 B   18 MiB  100 GiB  0.99  1.00    1      up
 4    ssd  0.09859   1.00000  101 GiB  1.0 GiB  328 KiB   0 B   18 MiB  100 GiB  0.99  1.00    1      up
 5    ssd  0.09859   1.00000  101 GiB  1.0 GiB  360 KiB   0 B   22 MiB  100 GiB  0.99  1.00    1      up
```

Expected results:
pg_num_check() should disallow the creation of this pool (since it's pg num will exceed the per_osd limit with this crush rule)

```
Error ERANGE: pg_num 256 size 3 for this pool would result in 256 cumulative PGs per OSD (768 total PG replicas on 3 'in' root OSDs by crush rule) which exceeds the mon_max_pg_per_osd value of 250
```

Additional info:
`ceph osd tree` notice the new crush rule named: `osd_test`.

```
ID  CLASS  WEIGHT   TYPE NAME         STATUS  REWEIGHT  PRI-AFF
-5         0.19717  root osd_test
0    ssd  0.09859      osd.0             up   1.00000  1.00000
-1         0.59151  root default
-3         0.59151      host folio
0    ssd  0.09859          osd.0         up   1.00000  1.00000
1    ssd  0.09859          osd.1         up   1.00000  1.00000
2    ssd  0.09859          osd.2         up   1.00000  1.00000
3    ssd  0.09859          osd.3         up   1.00000  1.00000
4    ssd  0.09859          osd.4         up   1.00000  1.00000
5    ssd  0.09859          osd.5         up   1.00000  1.00000
```