NFSv42: Don't fail clone() unless the OP_CLONE operation failed

The failure to retrieve post-op attributes has no bearing on whether or
not the clone operation itself was successful. We must therefore ignore
the return value of decode_getfattr() when looking at the success or
failure of nfs4_xdr_dec_clone().

Fixes: 36022770

 ("nfs42: add CLONE xdr functions")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>