uprobes/core: Allocate XOL slots for uprobes use

Uprobes executes the original instruction at a probed location
out of line. For this, we allocate a page (per mm) upon the
first uprobe hit, in the process user address space, divide it
into slots that are used to store the actual instructions to be
singlestepped. These slots are known as xol (execution out of
line) slots.

Care is taken to ensure that the allocation is in an unmapped
area as close to the top of the user address space as possible,
with appropriate permission settings to keep selinux like
frameworks happy.

Upon a uprobe hit, a free slot is acquired, and is released
after the singlestep completes.

Lots of improvements courtesy suggestions/inputs from Peter and
Oleg.

[ Folded a fix for build issue on powerpc fixed and reported by
  Stephen Rothwell. ]

Signed-off-by: Srikar Dronamraju <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Ananth N Mavinakayanahalli <[email protected]>
Cc: Jim Keniston <[email protected]>
Cc: Linux-mm <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Anton Arapov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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