s3/locking: tweak share_entry_forall()

s3/locking: tweak share_entry_forall()

Gives the callback function more control about when of a locking.tdb
traverse is stopped. If the callback function returns:

  0 => continue traverse
  1 => stop loop over share_mode_entries, but continue share_mode_data traverse
 -1 => stop whole share_mode_data traverse

All existing callers of share_entry_forall() either return 0 or -1 from their
callback functions, so no change in behaviour.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15608

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>