misc: fastrpc: Rework fastrpc_req_munmap

misc: fastrpc: Rework fastrpc_req_munmap

Move the lookup of the munmap request to the fastrpc_req_munmap and pass
on only the buf to the lower level fastrpc_req_munmap_impl. That way
we can use the lower level fastrpc_req_munmap_impl on error path in
fastrpc_req_mmap to free the buf without searching for the munmap
request it belongs to.

Co-developed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20221125071405.148786-7-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>