qa/standalone/mon/osd-pool-df.sh: fix test to check for the right values

qa/standalone/mon/osd-pool-df.sh: fix test to check for the right values

Though the test passed, we weren't checking for the correct values:

.../qa/standalone/mon/osd-pool-df.sh:62: TEST_ceph_df:  ceph df -f json
.../qa/standalone/mon/osd-pool-df.sh:62: TEST_ceph_df:  jq .stats.total_avail_bytes
../qa/standalone/mon/osd-pool-df.sh:62: TEST_ceph_df:  local global_avail=0
.../qa/standalone/mon/osd-pool-df.sh:63: TEST_ceph_df:  ceph df -f json
.../qa/standalone/mon/osd-pool-df.sh:63: TEST_ceph_df:  jq '.pools | map(select(.name == "$rep_poolname"))[0].stats.max_avail'
../qa/standalone/mon/osd-pool-df.sh:63: TEST_ceph_df:  local rep_avail=null
.../qa/standalone/mon/osd-pool-df.sh:64: TEST_ceph_df:  ceph df -f json
.../qa/standalone/mon/osd-pool-df.sh:64: TEST_ceph_df:  jq '.pools | map(select(.name == "$ec_poolname"))[0].stats.max_avail'
../qa/standalone/mon/osd-pool-df.sh:64: TEST_ceph_df:  local ec_avail=null
../qa/standalone/mon/osd-pool-df.sh:66: TEST_ceph_df:  echo '0 >= null*3'
../qa/standalone/mon/osd-pool-df.sh:66: TEST_ceph_df:  bc
1
../qa/standalone/mon/osd-pool-df.sh:67: TEST_ceph_df:  echo '0 >= null*1.5'
../qa/standalone/mon/osd-pool-df.sh:67: TEST_ceph_df:  bc
1

Signed-off-by: Neha Ojha <nojha@redhat.com>