staging/lustre: Always return EEXIST on mkdir for existing names

if the name already exists, but we don't have write permissions
in the parent, force talking to the MDS to determine what
more sensical error code to return.
This also happens to fix matlab and other such programs that
assume that EEXIST is the only valid error code for mkdir of
an existing directory.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>