Remove lwp -> pid conversion in linux_nat_xfer_partial

The linux_nat_xfer_partial does a conversion of inferior_ptid: if it's
an LWP (ptid::lwp != 0), it builds a new ptid with the lwp as
the pid and assigns that temporarily to inferior_ptid.  For example, if
inferior_ptid is:

  { .pid = 1234, .lwp = 1235 }

it will assign this to inferior_ptid for the duration of the call:

  { .pid = 1235, .lwp = 0 }

Instead of doing this, this patch teaches the inf-ptrace implementation
of xfer_partial to deal with ptids representing lwps by using
get_ptrace_pid.

Also, in linux_proc_xfer_spu and linux_proc_xfer_partial, we use ptid_get_lwp
instead of ptid_get_pid.  While not strictly necessary, since the content of
/proc/<pid> and /proc/<lwp> should be the same, it's a bit safer, because:

- some files under /proc/<pid>/ may not work if the <pid> thread is
  running, just like ptrace requires a stopped thread.  The current
  thread's lwp id is more likely to be in the necessary state (stopped).

- if the leader (<pid>) had exited and is thus now zombie, then several
  files under "/proc/<pid>" won't work, while they will if you use
  "/proc/<lwp>".

The testsuite found no regression on native amd64 linux.

gdb/ChangeLog:

	* inf-ptrace.c (inf_ptrace_xfer_partial): Get pid from ptid
	using get_ptrace_pid.
	* linux-nat.c (linux_nat_xfer_partial): Don't set/restore
	inferior_ptid.
	(linux_proc_xfer_partial, linux_proc_xfer_spu): Use lwp of
	inferior_ptid instead of pid.

1 files changed, 1 insertions, 1 deletions

diff --git a/gdb/inf-ptrace.c b/gdb/inf-ptrace.c
index 61d2426..f912d28 100644
--- a/gdb/inf-ptrace.c
+++ b/gdb/inf-ptrace.c
@@ -520,7 +520,7 @@ inf_ptrace_xfer_partial (struct target_ops *ops, enum target_object object,
 			 const gdb_byte *writebuf,
 			 ULONGEST offset, ULONGEST len, ULONGEST *xfered_len)
 {
-  pid_t pid = ptid_get_pid (inferior_ptid);
+  pid_t pid = get_ptrace_pid (inferior_ptid);
 
   switch (object)
     {