posix-acl: fixup extended ACL entries properly

Typically when updating cached ACL from backend, we get both iatt and
ACL xattrs (like lookup, readdirplus etc.) However in calls like
setattr(), the mode would have updated but we receive only iatt and not
the ACL xattrs. In such case we need to "spread" the effects of the
changed mode properly into the cached ACL xattr ourselves.

Change-Id: I23a7bc9c14722ff6848e175ed4bbe863a21ce2c9
BUG: 998967
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/6002
Tested-by: Gluster Build System <jenkins@build.gluster.com>