NFSD: Rename nfsd4_encode_dirent()

Rename nfsd4_encode_dirent() to match the naming convention already
used in the NFSv2 and NFSv3 readdir paths. The new name reflects the
name of the spec-defined XDR data type for an NFSv4 directory entry.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>