mm: enforce __must_check on VMA merge and split

It is of critical importance to check the return results on VMA merge (and
split), failure to do so can result in use-after-free's.  This bug has
recurred, so have the compiler enforce this check to prevent any future
repetition.

Link: https://lkml.kernel.org/r/20241206225036.273103-1-lorenzo.stoakes@oracle.com


Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>