gdb/linux-nat: add pid parameter to linux_proc_xfer_memory_partial

Add a pid parameter to linux_proc_xfer_memory_partial, making the
inferior_ptid reference bubble up close to the target_ops::xfer_partial
boundary.  No behavior change expected.

Change-Id: I58171b00ee1bba1ea22efdbb5dcab8b1ab3aac4c