percpu: remove @may_alloc from pcpu_get_pages()

pcpu_get_pages() creates the temp pages array if not already allocated
and returns the pointer to it.  As the function is called from both
[de]population paths and depopulation can only happen after at least
one successful population, the param doesn't make any difference - the
allocation will always happen on the population path anyway.

Remove @may_alloc from pcpu_get_pages().  Also, add an lockdep
assertion pcpu_alloc_mutex instead of vaguely stating that the
exclusion is the caller's responsibility.

Signed-off-by: Tejun Heo <tj@kernel.org>