soc: qcom: mem-buf: Fix memory leak in mem_buf_alloc()

When mem_buf_alloc() encounters an error after mapping the memory
in the stage-2 page tables of the VM, the SGL descriptor is not
freed in the error handling path, so free it.

Change-Id: I6e6fa296311575bb55cd0b9da7a138970e8b8e74
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>