pnfs: save layoutcommit cred at layout header init

commit 9fa40758 upstream.

No need to save it for every lseg.
No need to save it at every pnfs_set_layoutcommit.

[Needed in v3.0]
Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Jim Rees <rees@umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>