qa/tasks/backfill_toofull.py: Fix assert failures with & without compression

qa/tasks/backfill_toofull.py: Fix assert failures with & without compression

The following issues with the test are addressed:

1. The test was encountering assertion failure (assert backfillfull < 0.9) with
   compression enabled. This was because the condition was not factoring in the
   compression ratio. Without it the backfillfull ratio can easily exceed 1. By
   factoring in the compression ratio, the backfillfull ratio will be in the
   range (0 - n), where n can vary depending on the type of compression used.

2. The main contributing factor for (1) above is the amount of data written to
   the pool. The writes were time-bound earlier leading to excess data and
   eventually the assertion failure. By limiting the data written to the OSDs
   to 50% of the OSD capacity in the first phase and only 20% in the re-write
   phase, the outcome of the test is more deterministic regardless of
   compression being enabled or not.

3. A potential false cluster error is avoided by swapping the setting of
   the nearfull-ratio and backfill-ratio after the re-write phase.

4. Fix a couple of typos - s/tartget/target.

Fixes: https://tracker.ceph.com/issues/71005
Signed-off-by: Sridhar Seshasayee <sridhar.seshasayee@ibm.com>
(cherry picked from commit 91de6a0b7b8b8c2531446555c25bf53e23635982)