Call the right helper function when setting mallopt M_ARENA_MAX (BZ #21338)

Fixes a typo introduced in commit
be7991c0. This caused
mallopt(M_ARENA_MAX) as well as the environment variable
MALLOC_ARENA_MAX to not work as intended because it set the
wrong internal parameter.

 	[BZ #21338]
	* malloc/malloc.c: Call do_set_arena_max for M_ARENA_MAX
	instead of incorrect do_set_arena_test