mm/mremap: convert vma_adjust() to vma_expand()

Stop using vma_adjust() in preparation for removing the function.  Export
vma_expand() to use instead.

Link: https://lkml.kernel.org/r/20230120162650.984577-45-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>