nfs/localio: refactor iocb and iov_iter_bvec initialization

nfs/localio: refactor iocb and iov_iter_bvec initialization

nfs_local_iter_init() is updated to follow the same pattern to
initializing LOCALIO's iov_iter_bvec as was established by
nfsd_iter_read().

Other LOCALIO iocb initialization refactoring in this commit offers
incremental cleanup that will be taken further by the next commit.

No functional change.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>