nfsd: fix nfs4_file refcount leak in nfsd_get_dir_deleg()

In the Linux kernel, the following vulnerability has been resolved: nfsd: fix nfs4_file refcount leak in nfsd_get_dir_deleg() Claude pointed out that there is a nfs4_file refcount leak in nfsd_get_dir_deleg(). Ensure that the reference to "fp" is released before returning.