tools/oxenstored: Fix quota calculation for mkdir EEXIST

tools/oxenstored: Fix quota calculation for mkdir EEXIST

We increment the domain's quota on mkdir even when the node already exists.
This results in a quota inconsistency after live update, where reconstructing
the tree from scratch results in a different quota.

Not a security issue because the domain uses up quota faster, so it will only
get a Quota error sooner than it should.

Found by the structured fuzzer.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>