UPSTREAM: rss_stat: Add support to detect RSS updates of external mm

When a process updates the RSS of a different process, the rss_stat
tracepoint appears in the context of the process doing the update. This
can confuse userspace that the RSS of process doing the update is
updated, while in reality a different process's RSS was updated.

This issue happens in reclaim paths such as with direct reclaim or
background reclaim.

This patch adds more information to the tracepoint about whether the mm
being updated belongs to the current process's context (curr field). We
also include a hash of the mm pointer so that the process who the mm
belongs to can be uniquely identified (mm_id field).

Also vsprintf.c is refactored a bit to allow reuse of hashing code.

Change-Id: Ic87af93af608c83be0b08757aed99d2b9c2c01d8
Reported-by: Ioannis Ilkos <ilkos@google.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Petr Mladek <pmladek@suse.com> # lib/vsprintf.c
Signed-off-by: Joel Fernandes <joelaf@google.com>