llkd: Add __get_user_page stack symbol checking

Add ro.llk.stack to list a set of symbols that should rarely happen
but if persistent in multiple checks, indicates a live lock condition.
At ro.llk.stack.timeout_ms the process is sent a kill, if it remains,
then panic the kernel.

There is no ABA detection in the paths, the condition for the
stack symbol being present instantaneously must be its rarity of
being caught.  If a livelock occurs in the path of the symbol, then
it is possible more than one path could be stuck in the state, but
the best candidate symbols are found underneath a lock resulting in
only one process being the culprit, and the best aim.  There may be
processes that induce a look of persistence, if so the symbol is not
a candidate for checking.

The current candidate is __get_user_pages, after mm is locked,
should be a very short reference to look up a page, but can be
longer term if starved, or a condition causes a conflicting loop.

Test: compile
Bug: 33808187
Change-Id: I946e85641e59229b7491e929fcab5f1240794254