All callers now have a folio and can call mlock_vma_folio().  Update the
documentation to refer to mlock_vma_folio().

Link: https://lkml.kernel.org/r/20230116192827.2146732-3-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>