pNFS: Prevent the layout header refcount going to zero in pnfs_roc()

pNFS: Prevent the layout header refcount going to zero in pnfs_roc()

BugLink: http://bugs.launchpad.net/bugs/1756100
commit 9c6376ebddad585da4238532dd6d90ae23ffee67 upstream.

Ensure that we hold a reference to the layout header when processing
the pNFS return-on-close so that the refcount value does not inadvertently
go to zero.

Reported-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org # v4.10+
Tested-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>