mm: move dup_mmap() to mm

This is a key step in our being able to abstract and isolate VMA
allocation and destruction logic.

This function is the last one where vm_area_free() and vm_area_dup() are
directly referenced outside of mmap, so having this in mm allows us to
isolate these.

We do the same for the nommu version which is substantially simpler.

We place the declaration for dup_mmap() in mm/internal.h and have
kernel/fork.c import this in order to prevent improper use of this
functionality elsewhere in the kernel.

While we're here, we remove the useless #ifdef CONFIG_MMU check around
mmap_read_lock_maybe_expand() in mmap.c, mmap.c is compiled only if
CONFIG_MMU is set.

Link: https://lkml.kernel.org/r/e49aad3d00212f5539d9fa5769bfda4ce451db3e.1745853549.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Suggested-by: Pedro Falcato <[email protected]>
Reviewed-by: Pedro Falcato <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Reviewed-by: Suren Baghdasaryan <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Kees Cook <[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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