misc: fastrpc: Fix dma_buf object leak in fastrpc_map_lookup

In fastrpc_map_lookup, dma_buf_get is called to obtain a reference to
the dma_buf for comparison purposes. However, this reference is never
released when the function returns, leading to a dma_buf memory leak.

Fix this by adding dma_buf_put before returning from the function,
ensuring that the temporarily acquired reference is properly released
regardless of whether a matching map is found.

Fixes: 9031626ade ("misc: fastrpc: Fix fastrpc_map_lookup operation")
Cc: stable@kernel.org
Signed-off-by: Junhao Xie <bigfoot@radxa.com>
Tested-by: Xilin Wu <sophon@radxa.com>
Rule: add
Link: https://lore.kernel.org/stable/48B368FB4C7007A7%2B20251017083906.3259343-1-bigfoot%40radxa.com
Link: https://patch.msgid.link/48B368FB4C7007A7+20251017083906.3259343-1-bigfoot@radxa.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/fastrpc.c

@@ -381,6 +381,8 @@ static int fastrpc_map_lookup(struct fastrpc_user *fl, int fd,
 	}
 	spin_unlock(&fl->lock);
 
+	dma_buf_put(buf);
+
 	return ret;
 }