nfsd: put the export reference in nfsd4_verify_deleg_dentry

nfsd_lookup_dentry returns an export reference in addition to the dentry
ref. Ensure that we put it too.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2138866


Fixes: 876c553c ("NFSD: verify the opened dentry after setting a delegation")
Reported-by: Yongcheng Yang <yoyang@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>