CVE-2026-74647: misc: fastrpc: Remove buffer from list prior to unmap operation

In the Linux kernel, the following vulnerability has been resolved:

misc: fastrpc: Remove buffer from list prior to unmap operation

fastrpcreqmunmapimpl() is called to unmap any buffer. The buffer is getting removed from the list after it is unmapped from DSP. This can create potential race conditions if multiple threads invoke unmap concurrently, where one thread may remove the entry from the list while another thread's unmap operation is still ongoing.

Fix this by removing the buffer entry from the list before calling the unmap operation. If the unmap fails, the entry is re-added to the list so that userspace can retry the unmap, or alternatively, the buffer will be cleaned up during device release when the DSP process is torn down and all DSP-side mappings are freed along with remaining buffers in the list.