pNFS: Stricter ordering of layoutget and layoutreturn

If a layout return is in progress, we should wait for it to complete,
in case the layout segment we are picking up gets returned too.

Fixes: 30cb3ee2

 ("pNFS: Handle NFS4ERR_OLD_STATEID on layoutreturn by bumping the state seqid")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>