selinux: revert our use of vma_is_initial_heap()

Unfortunately it appears that vma_is_initial_heap() is currently broken
for applications that do not currently have any heap allocated, e.g.
brk == start_brk.  The breakage is such that it will cause SELinux to
check for the process/execheap permission on memory regions that cross
brk/start_brk even when there is no heap.

The proper fix would be to correct vma_is_initial_heap(), but as there
are multiple callers I am hesitant to unilaterally modify the helper
out of concern that I would end up breaking some other subsystem.  The
mm developers have been made aware of the situation and hopefully they
will have a fix at some point in the future, but we need a fix soon so
we are simply going to revert our use of vma_is_initial_heap() in favor
of our old logic/code which works as expected, even in the face of a
zero size heap.  We can return to using vma_is_initial_heap() at some
point in the future when it is fixed.

Cc: [email protected]
Reported-by: Marc Reisner <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]
Fixes: 68df1baf158f ("selinux: use vma_is_initial_stack() and vma_is_initial_heap()")
Signed-off-by: Paul Moore <[email protected]>

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