s3: winbind: Remove fstring from wb_acct_info struct

s3: winbind: Remove fstring from wb_acct_info struct

The group enumeration backend functions try to allocate an array of
wb_acct_info structs with a number of elements equal to the number of
groups. In domains with a large number of groups this allocation may
fail due to the size of the chunk.

Found while trying to enumerate the groups in a domain with more than
700k groups.

Signed-off-by: Samuel Cabrero <scabrero@suse.de>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>