misc: fastrpc: Fix double free of 'buf' in error path

smatch warning:
drivers/misc/fastrpc.c:1926 fastrpc_req_mmap() error: double free of 'buf'

In fastrpc_req_mmap() error path, the fastrpc buffer is freed in
fastrpc_req_munmap_impl() if unmap is successful.

But in the end, there is an unconditional call to fastrpc_buf_free().
So the above case triggers the double free of fastrpc buf.

Fixes: 72fa6f78 ("misc: fastrpc: Rework fastrpc_req_munmap")
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Sukrut Bellary <sukrut.bellary@linux.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/20240902141409.70371-2-srinivas.kandagatla@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>