pNFS: Fix a hang in nfs4_evict_inode()

We are not allowed to call pnfs_mark_matching_lsegs_return() without
also holding a reference to the layout header, since doing so could lead
to the reference count going to zero when we call
pnfs_layout_remove_lseg(). This again can lead to a hang when we get to
nfs4_evict_inode() and are unable to clear the layout pointer.

pnfs_layout_return_unused_byserver() is guilty of this behaviour, and
has been seen to trigger the refcount warning prior to a hang.

Fixes: b6d49ecd ("NFSv4: Fix a pNFS layout related use-after-free race when freeing the inode")
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>