mm: optimise vmf_anon_prepare() for VMAs without an anon_vma

mm: optimise vmf_anon_prepare() for VMAs without an anon_vma

If the mmap_lock can be taken for read, we can call __anon_vma_prepare()
while holding it, saving ourselves a trip back through the fault handler.

Link: https://lkml.kernel.org/r/20240426144506.1290619-5-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>