NFSv4: Fix a pNFS layout related use-after-free race when freeing the inode

[ Upstream commit b6d49ecd ]

When returning the layout in nfs4_evict_inode(), we need to ensure that
the layout is actually done being freed before we can proceed to free the
inode itself.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>