lib/debug: set the correct default backend loglevel to MAX_DEBUG_LEVEL

lib/debug: set the correct default backend loglevel to MAX_DEBUG_LEVEL

This fixed a regression introduced by commit
c83ce5f4f99aef94530411ec82cc03e9935b352d for bug #14345.

The backend loglevel globally restricts logging of a particular backend. If this
value is smaller then any explicitly configured logging class, logging for this
class is skipped.

Eg, given the following logging config in smb.conf:

  log level = 1 auth_json_audit:3@/var/log/samba/samba_auth_audit.log

the default class loglevel of 1 (dbgc_config[DBGC_ALL].loglevel) will be
assigned to the backend loglevel.

So even though the logging class auth_json_audit is configured at level 3, this
doesn't become effective as the file backend drops all log messages with a level
below 1.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14426

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu Jul  2 13:25:29 UTC 2020 on sn-devel-184