NFSD: Move fill_pre_wcc() and fill_post_wcc()

These functions are related to file handle processing and have
nothing to do with XDR encoding or decoding. Also they are no longer
NFSv3-specific. As a clean-up, move their definitions to a more
appropriate location. WCC is also an NFSv3-specific term, so rename
them as general-purpose helpers.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>