NFSv4.1: Simplify layout return in pnfs_layout_process()

If the server hands us a layout that does not match the one we currently
hold, then have pnfs_mark_matching_lsegs_return() just ditch the old
layout if NFS_LSEG_LAYOUTRETURN is not set.

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