[Bug 273962] copy_file_range does not work on shared memory objects

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273962

--- Comment #2 from David Chisnall <theraven@FreeBSD.org> ---
(In reply to Rick Macklem from comment #1)
> The title uses "file" which I think of as "regular file",

On *NIX, everything is a file.  I found it very surprising that it didn't work
with shared memory objects.  On Linux (where this call originated), shared
memory objects are files in a special memory filesystem and so the line is
especially blurry.  I don't believe the Linux implementation has such a
limitation: it falls back to the equivalent of a read and write loop in the
kernel if the fast paths are not supported).

As a user, it's very surprising that a pair of file descriptors that work with
lseek, read, and write, do not work with `copy_file_range`.  It's made worse by
the fact that `EINVAL` covers a variety of errors and so writing fallback code
for when the file descriptor provided to an API does not work with this
function cannot unambiguously detect that the reason for the failure was a file
descriptor for which this is not supported.

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