smbd: dfree - ignore quota if not enforced

smbd: dfree - ignore quota if not enforced

When calculating free disk space, do not take user quota
into account if quota is globally not enforced on the file
system.

This is meant to fix a specific problem with XFS. One might
say "why don't you fix the XFS-specific code instead?". The
reason for that is that getting and setting quota must not
be affected by whether quota is actually enforced. NTFS has
the same notion of separating quota accounting (and being
able to configure / retrieve configured quota), from quota
enforcement.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11937

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sat May 28 00:09:05 CEST 2016 on sn-devel-144