mm: refactor __mmap_region()

We have seen bugs and resource leaks arise from the complexity of the
__mmap_region() function.  This, and the generally deeply fragile error
handling logic and complexity which makes understanding the function
difficult make it highly desirable to refactor it into something readable.

Achieve this by separating the function into smaller logical parts which
are easier to understand and follow, and which importantly very
significantly simplify the error handling.

Note that we now call vms_abort_munmap_vmas() in more error paths than we
used to, however in cases where no abort need occur, vms->nr_pages will be
equal to zero and we simply exit this function without doing more than we
would have done previously.

Importantly, the invocation of the driver mmap hook via mmap_file() now
has very simple and obvious handling (this was previously the most
problematic part of the mmap() operation).

Use a generalised stack-based 'mmap state' to thread through values and
also retrieve state as needed.

Also avoid ever relying on vma merge (vmg) state after a merge is
attempted, instead maintain meaningful state in the mmap state and
establish vmg state as and when required.

This avoids any subtle bugs arising from merge logic mutating this state
and mmap_region() logic later relying upon it.

Link: https://lkml.kernel.org/r/25bd2edc3275450f448cbfe0756ce2a7cd06810f.1729858176.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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