fs/9p: Fix the return error on default acl removal

If we don't have default ACL, then trying to remove
default acl on a file should return 0.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>