mm/mmap: convert do_brk_flags() to use vma_prepare() and vma_complete()

Use the abstracted vma locking for do_brk_flags()

Link: https://lkml.kernel.org/r/20230120162650.984577-48-Liam.Howlett@oracle.com


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