[RFC V5] Add gup trace points support

[Yang Shi]

v5:
* Fixed a typo introduced by v4 rebase
* Removed redundant "#define CREATE_TRACE_POINTS" from architecture specifc
  gup.c

v4:
* Adopted Steven's suggestion to use "unsigned int" for nr_pages to save
  space in ring buffer since it is unlikely to have more than 0xffffffff
  pages are touched by gup in one invoke
* Remove unnecessray type cast

v3:
* Adopted suggestion from Dave Hansen to move the gup header include to the last
* Adopted comments from Steven:
  - Use DECLARE_EVENT_CLASS and DEFINE_EVENT
  - Just keep necessary TP_ARGS
* Moved archtichture specific fall-backable fast version trace point after the
  do while loop since it may jump to the slow version.
* Not implement recording return value since Steven plans to have it in generic
  tracing code
 
v2:
* Adopted commetns from Steven
  - remove all reference to tsk->comm since it is unnecessary for non-sched
    trace points
  - reduce arguments for __get_user_pages trace point and update mm/gup.c
    accordingly
* Added Ralf's acked-by for patch 4/7.


Some background about why I think this might be useful.

When I was profiling some hugetlb related program, I got page-faults event
doubled when hugetlb is enabled. When I looked into the code, I found page-faults
come from two places, do_page_fault and gup. So, I tried to figure out which
play a role (or both) in my use case. But I can't find existing finer tracing
event for sub page-faults in current mainline kernel.

So, I added the gup trace points support to have finer tracing events for
page-faults. The below events are added:

__get_user_pages
__get_user_pages_fast
fixup_user_fault

Both __get_user_pages and fixup_user_fault call handle_mm_fault.

Just added trace points to raw version __get_user_pages since all variants
will call it finally to do real work.

Although __get_user_pages_fast doesn't call handle_mm_fault, it might be useful
to have it to distinguish between slow and fast version.
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