Populate post_op_attr with updated attributes after commit (2.0)

This commit replaces:

  commit 75eae985f14c0236beabf0fa3727469f289ec8f4
  Author: Srikrishan Malik <srimalik@in.ibm.com>
  Date:   Thu Feb 7 19:07:36 2013 +0530

      Populate post_op_attr with updated attributes after commit.

  Remove incorrect SetPreAttr.  Don't need post_attr.
Remove wcc pre-attrs form Write and Commit Replies

Replaces:

  commit cc3af75fa4e8bd1668f0a967f6e7fb0a03261b20
  Author: Venkateswararao Jujjuri (JV) <jvrao@linux.vnet.ibm.com>
  Date:   Fri Mar 8 18:22:48 2013 -0800

    Remove wcc pre-attrs form Write and Commit Replies

    Given that Ganesha need to make multiple calls to underlying filesystem
    to perform write and then to get attributes, there is no way we can provide
    accurate/atomic wcc pre-attrs. Because of this, the attr times on the client
    side are not matching, resulting in invalidating client's cache. Removing
    wcc pre-attrs in the Write and Commit replies, allows client to accurately
    follow close-to-open semantics and keep the client cache sane.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Update 15to20-porting.txt.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>