CVE-2020-10704: smb.conf: Add max ldap request sizes

CVE-2020-10704: smb.conf: Add max ldap request sizes

Add two new smb.conf parameters to control the maximum permitted ldap
request size.

Adds:
   ldap max anonymous request size       default 250Kb
   ldap max authenticated request size   default 16Mb

Credit to OSS-Fuzz

REF: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20454
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14334

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>