mgr: forward the root-logger fallback at the record's own level

ceph_module.mgr_log() emitted every record at dout(0): a record either
passed the Python-side gate and went unconditionally to both the log
file and the in-memory recent buffer, or never reached the C++ log at
all.  The two-level debug_mgr semantics (file level / memory level)
never applied, so capturing third-party DEBUG logs meant sending each
record to the log file and a log_max_recent slot; with the kubernetes
client logging multi-megabyte HTTP response bodies at DEBUG, the
recent buffer alone could grow to tens of gigabytes.

Pass the record's level through mgr_log() and emit at the mapped ceph
level: CRITICAL/ERROR at 0, WARNING at 1, INFO at 2, DEBUG at 20.
INFO maps to 2 so it stays visible in the log file at the default
debug_mgr=2/5; DEBUG maps to 20 so floods are dropped there, while
debug_mgr=2/20 captures third-party DEBUG in the in-memory buffer
without touching the log file.  The Python-side gate follows the
higher of the two debug_mgr levels so memory-only records still
reach the C++ side.

Fixes: https://tracker.ceph.com/issues/77633
Signed-off-by: Kefu Chai <k.chai@proxmox.com>