NFSv4.1: Rename the pnfs_put_lseg_common to pnfs_layout_remove_lseg

The latter name is more descriptive of the actual function.
Also rename pnfs_insert_layout to pnfs_layout_insert_lseg.

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