NFSv4: Fix memory leak in nfs4_set_security_label

[ Upstream commit aad11473

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We leak nfs_fattr and nfs4_label every time we set a security xattr.

Signed-off-by: Dmitry Mastykin <mastichi@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>