lib/xz: Move s->lzma.len = 0 initialization to lzma_reset()

lib/xz: Move s->lzma.len = 0 initialization to lzma_reset()

It's a more logical place even if the resetting needs to be done
only once per LZMA2 stream (if lzma_reset() called in the middle
of an LZMA2 stream, .len will already be 0).

Link: https://lore.kernel.org/r/20211010213145.17462-4-xiang@kernel.org
Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>