NFSv4: Fixes for nfs4_inode_return_delegation()

commit 6e176d47 upstream.

We mustn't call nfs_wb_all() on anything other than a regular file.
Furthermore, we can exit early when we don't hold a delegation.

Reported-by: David Wysochanski <dwysocha@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>