mm: only enforce minimum stack gap size if it's sensible

The generic mmap_base code tries to leave a gap between the top of the
stack and the mmap base address, but enforces a minimum gap size (MIN_GAP)
of 128MB, which is too large on some setups.  In particular, on arm tasks
without ADDR_LIMIT_32BIT, the STACK_TOP value is less than 128MB, so it's
impossible to fit such a gap in.

Only enforce this minimum if MIN_GAP < MAX_GAP, as we'd prefer to honour
MAX_GAP, which is defined proportionally, so scales better and always
leaves us with both _some_ stack space and some room for mmap.

This fixes the usercopy KUnit test suite on 32-bit arm, as it doesn't set
any personality flags so gets the default (in this case 26-bit) task size.
This test can be run with: ./tools/testing/kunit/kunit.py run --arch arm
usercopy --make_options LLVM=1

Link: https://lkml.kernel.org/r/[email protected]
Fixes: dba79c3df4a2 ("arm: use generic mmap top-down layout and brk randomization")
Signed-off-by: David Gow <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Alexandre Ghiti <[email protected]>
Cc: Linus Walleij <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Russell King <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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