fs: sort out a stale comment about races between fd alloc and dup2

It claims the issue is only relevant for shared descriptor tables which
is of no concern for POSIX (but then is POSIX of concern to anyone
today?), which I presume predates standarized threading.

The comment also mentions the following systems:
- OpenBSD installing a larval file -- they moved away from it, file is
  installed late and EBUSY is returned on conflict
- FreeBSD returning EBADF -- reworked to install the file early like
  OpenBSD used to do
- NetBSD "deadlocks in amusing ways" -- their solution looks
  Solaris-inspired (not a compliment) and I would not be particularly
  surprised if it indeed deadlocked, in amusing ways or otherwise

I don't believe mentioning any of these adds anything and the statement
about the issue not being POSIX-relevant is outdated.

dup2 description in POSIX still does not mention the problem.

Just shorten the comment and be done with it.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20241205154743.1586584-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

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