fix Mmap.sync! for array mapped from an offset

This fixes error when calling `Mmap.sync!` on an array that is not memory mapped from a pagesized offset of a file.

````
julia> using Base.Mmap

julia> f = open("arrayfile", "r+");

julia> A = Mmap.mmap(f, Vector{Int64}, (200,), 0);

julia> Mmap.sync!(A)

julia> B = Mmap.mmap(f, Vector{Int64}, (200,), 8);

julia> Mmap.sync!(B)
ERROR: SystemError: msync: Invalid argument
 [inlined code] from ./int.jl:33
 in sync!(Base.Mmap.#sync!, Array{Int64,1}, Int64) at ./mmap.jl:207 (repeats 2 times)
 in eval at ./boot.jl:267
````

Since `Mmap.mmap` maps from the beginning of a page boundary, `pointer(A)` is not at the page boundary when an offset is provided.
But since the pointer returned from `mmap` is aligned at page boundary, `sync!` now recalculates the offset before calling `msync!`.

````
julia> using Base.Mmap

julia> f = open("arrayfile", "r+");

julia> B = Mmap.mmap(f, Vector{Int64}, (200,), 8);

julia> Mmap.sync!(B)
````

base/mmap.jl

@@ -204,9 +204,11 @@ const MS_INVALIDATE = 2
 const MS_SYNC = 4
 
 function sync!{T}(m::Array{T}, flags::Integer=MS_SYNC)
-    @unix_only systemerror("msync", ccall(:msync, Cint, (Ptr{Void}, Csize_t, Cint), pointer(m), length(m)*sizeof(T), flags) != 0)
+    offset = rem(UInt(pointer(m)), PAGESIZE)
+    ptr = pointer(m) - offset
+    @unix_only systemerror("msync", ccall(:msync, Cint, (Ptr{Void}, Csize_t, Cint), ptr, length(m)*sizeof(T), flags) != 0)
     @windows_only systemerror("could not FlushViewOfFile: $(Libc.FormatMessage())",
-                    ccall(:FlushViewOfFile, stdcall, Cint, (Ptr{Void}, Csize_t), pointer(m), length(m)) == 0)
+                    ccall(:FlushViewOfFile, stdcall, Cint, (Ptr{Void}, Csize_t), ptr, length(m)) == 0)
 end
 sync!(B::BitArray, flags::Integer=MS_SYNC) = sync!(B.chunks, flags)
 

test/mmap.jl

@@ -278,3 +278,14 @@ n = similar(m, 12)
 @test length(n) == 12
 @test size(n) == (12,)
 finalize(m); m = nothing; gc()
+
+# test #14885
+file = tempname()
+touch(file)
+open(file, "r+") do s
+    A = Mmap.mmap(s, Vector{UInt8}, (10,), 0);
+    Mmap.sync!(A)
+    A = Mmap.mmap(s, Vector{UInt8}, (10,), 1);
+    Mmap.sync!(A)
+end
+rm(file)