NFSv4.1: Remove redundant reference to the pnfs_layout_hdr

Each layout segment already holds a reference to the pnfs_layout_hdr,
so there is no need to hold an extra reference that is released once
the last layout segment is freed.

Ensure that pnfs_find_alloc_layout() always returns a reference
to the pnfs_layout_hdr, which will be matched by the final call to
pnfs_put_layout_hdr() in pnfs_update_layout().

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>