mm/page_owner.c: remove unnecessary stack_trace field

mm/page_owner.c: remove unnecessary stack_trace field

Page owner uses the page_ext structure to keep meta-information for every
page in the system.  The structure also contains a field of type 'struct
stack_trace', page owner uses this field during invocation of the function
save_stack_trace.  It is easy to notice that keeping a copy of this
structure for every page in the system is very inefficiently in terms of
memory.

The patch removes this unnecessary field of page_ext and forces page owner
to use a stack_trace structure allocated on the stack.

[[email protected]: use struct initializers]
Signed-off-by: Sergei Rogachev <[email protected]>
Acked-by: Joonsoo Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>