pNFS: Layoutreturn must free the layout after the layout-private data

The layout-private data may depend on the layout and/or the inode
still existing when it does post-processing and frees its data, so we
need to free them after calling lrp->ld_private.ops->free().

This fixes a mirror list corruption issue in the flexfiles driver.

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