test: Thrasher: do not update pools_to_fix_pgp_num if nothing happens

test: Thrasher: do not update pools_to_fix_pgp_num if nothing happens

we should not update pools_to_fix_pgp_num if the pool is not expanded or
the pg_num is not increased due to pgs being created. this prevent us
from fixing the pgp_num after done with thrashing if we actually did
nothing when fixing the pgp_num when thrashing, but we removed the pool
from pools_to_fix_pgp_num after set_pool_pgpnum() returns.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit c0f0cde399a80cea617c115417e5390a63ec674e)