pNFS/flexfiles: Clean up redundant calls to pnfs_put_lseg()

Both nfs_pageio_reset_read_mds() and nfs_pageio_reset_write_mds()
do call pnfs_generic_pg_cleanup() for us.

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