pNFS: Fix a reference leak in _pnfs_return_layout

IF NFS_LAYOUT_RETURN_REQUESTED is not set, then we currently exit
without freeing the list of invalidated layout segments, leading
to a reference leak.

Reported-by: Olga Kornievskaia <aglo@umich.edu>
Fixes: 24408f52

 ("pNFS: Fix bugs in _pnfs_return_layout")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>