mm: mmap: fix documentation for vma_mas_szero

When the struct_mm input, mm, was changed to a struct ma_state, mas, the
documentation for the function was never updated.  This updates that
documentation reference.

Link: https://lkml.kernel.org/r/20221114003349.41235-1-ian@linux.cowan.aero


Signed-off-by: Ian Cowan <ian@linux.cowan.aero>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>