NFS: Fix 2 use after free issues in the I/O code

The writeback code wants to send a commit after processing the pages,
which is why we want to delay releasing the struct path until after
that's done.

Also, the layout code expects that we do not free the inode before
we've put the layout segments in pnfs_writehdr_free() and
pnfs_readhdr_free()

Fixes: 919e3bd9 ("NFS: Ensure we commit after writeback is complete")
Fixes: 4714fb51 ("nfs: remove pgio_header refcount, related cleanup")
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>