FROMLIST: ASoC: msm: qdspv2: add mutex lock when access output buffer length

Add mutex protection to avoid access output_len in parallel.

CRs-Fixed: 1104067
Bug: 33353700
Change-Id: I4e17258e2abee9cd68152f4b79520b00003aa80d
Signed-off-by: kunleiz <kunleiz@codeaurora.org>
Signed-off-by: Siqi Lin <siqilin@google.com>
(am from https://source.codeaurora.org/quic/la/kernel/msm-3.18/commit/?id=cb0701a2f99fa19f01fbd4249bda9a8eadb0241f)