NFS: Fix potential oops in nfs_inode_remove_request()

Once a folio's private data has been cleared, it's possible for another
process to clear the folio->mapping (e.g. via invalidate_complete_folio2
or evict_mapping_folio), so it wouldn't be safe to call
nfs_page_to_inode() after that.

Fixes: 0c493b5c ("NFS: Convert buffered writes to use folios")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Tested-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>