tools: testing: use existing atomic.h for vma/maple tests

tools: testing: use existing atomic.h for vma/maple tests

The shared userspace logic used for unit-testing maple tree and VMA code
currently has its own replacements for atomics helpers.  This is not
needed as the necessary APIs already have userspace implementations in the
tools tree.  Switching over to that allows deleting a bit of code.

Note that the implementation is different; while the version being deleted
here is implemented using liburcu, the existing version in tools uses
either x86 asm or compiler builtins.  It's assumed that both are equally
likely to be correct.

The tools tree's version of atomic_t is a struct type while the version
being deleted was just a typedef of an integer.  This means it's no longer
valid to call __sync_bool_compare_and_swap() directly on it.  One option
would be to just peek into the struct and call it on the field, but it
seems a little cleaner to just use the corresponding atomic.h API whic has
been added recently.  Now the fake mapping_map_writable() is copied from
the real one.

Link: https://lkml.kernel.org/r/20250828-b4-vma-no-atomic-h-v2-4-02d146a58ed2@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>