mm: perform VMA allocation, freeing, duplication in mm

Right now these are performed in kernel/fork.c which is odd and a
violation of separation of concerns, as well as preventing us from
integrating this and related logic into userland VMA testing going
forward.

There is a fly in the ointment - nommu - mmap.c is not compiled if
CONFIG_MMU not set, and neither is vma.c.

To square the circle, let's add a new file - vma_init.c.  This will be
compiled for both CONFIG_MMU and nommu builds, and will also form part of
the VMA userland testing.

This allows us to de-duplicate code, while maintaining separation of
concerns and the ability for us to userland test this logic.

Update the VMA userland tests accordingly, additionally adding a
detach_free_vma() helper function to correctly detach VMAs before freeing
them in test code, as this change was triggering the assert for this.

[[email protected]: remove stray newline, per Liam]
Link: https://lkml.kernel.org/r/f97b3a85a6da0196b28070df331b99e22b263be8.1745853549.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Reviewed-by: Pedro Falcato <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Suren Baghdasaryan <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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