From: NeilBrown <neilb@cse.unsw.edu.au>

Exclusive creates were failing with permission errors, because they don't set
the mode on the newly created file, and the permission check in fh_verify
(called at the end of do_open_lookup) was failing.  So pass in the
MAY_OWNER_OVERRIDE flag to allow the owner of the file to override the
permission check.