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)
````
(cherry picked from commit fc9fd22181664b4c61c52dfcf973ab919529bf49)
ref #14885
