nfs: rename pgio header ds_idx to ds_commit_idx

'ds_commit_idx' is a better name - it is used to select the right
commit bucket for pnfs.

Signed-off-by: Weston Andros Adamson <dros@primarydata.com>

diff --cc fs/nfs/direct.c
Simple merge

diff --cc fs/nfs/filelayout/filelayout.c
Simple merge

diff --cc include/linux/nfs_xdr.h
Simple merge