NFSv4.1: Balance pnfs_layout_hdr refcount in pnfs_layout_(insert|remove)_lseg

Ensure that the reference count for pnfs_layout_hdr reverts to the
original value after a call to pnfs_layout_remove_lseg().

Note that the caller is expected to hold a reference to the struct
pnfs_layout_hdr.

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