mmap: drop file-offset checking, defer to mmap(2) instead

[roypat]

When mmap-ing a file-like object, vm-memory was trying to validate the
the range [offset, offset+len) was valid in the file. However, our
homegrown check had lots of edge cases where it gave false-positives
(e.g. it rejected things that should really be working, such as vfio
devices, or fds that cannot be seeked like guest_memfd), and trying to
implement a check that works for all of these is in the end wasted
effort anyway, because the kernel validates all this as part of the mmap
syscall anyway. So just drop these checks in favor of failing at
mmap-time.

See also #195 (comment)

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