NFSv4/pnfs: pnfs_set_layout_stateid() should update the layout cred

If the cred assigned to the layout that we're updating differs from
the one used to retrieve the new layout segment, then we need to
update the layout plh_lc_cred field.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>