attach finalizer in mmap to the correct object (#54210)

Fixes #54128

stdlib/Mmap/src/Mmap.jl

@@ -256,7 +256,7 @@ function mmap(io::IO,
  end # os-test
  # convert mmapped region to Julia Array at `ptr + (offset - offset_page)` since file was mapped at offset_page
  A = unsafe_wrap(Array, convert(Ptr{T}, UInt(ptr) + UInt(offset - offset_page)), dims)
- finalizer(A) do x
+ finalizer(A.ref.mem) do x
  @static if Sys.isunix()
  systemerror("munmap", ccall(:munmap, Cint, (Ptr{Cvoid}, Int), ptr, mmaplen) != 0)
  else

stdlib/Mmap/test/runtests.jl

@@ -100,9 +100,9 @@ if !(Sys.ARCH === :powerpc64le || Sys.ARCH === :ppc64le)
  s = open(file, "r")
  m = mmap(s)
  @test_throws ReadOnlyMemoryError m[5] = UInt8('x') # tries to setindex! on read-only array
- finalize(m); m=nothing; GC.gc()
+ finalize(m); m=nothing;
 end
-
+GC.gc()
 write(file, "Hello World\n")
 
 s = open(file, "r")
@@ -336,8 +336,9 @@ open(file, "r+") do s
  finalize(A); A = nothing; GC.gc()
  A = mmap(s, Vector{UInt8}, (10,), 1)
  Mmap.sync!(A)
- finalize(A); A = nothing; GC.gc()
+ finalize(A); A = nothing;
 end
+GC.gc()
 rm(file)
 
 @testset "Docstrings" begin