fs: sort out fd allocation vs dup2 race commentary, take 2

fd_install() has a questionable comment above it.

While it correctly points out a possible race against dup2(), it states:
> We need to detect this and fput() the struct file we are about to
> overwrite in this case.
>
> It should never happen - if we allow dup2() do it, _really_ bad things
> will follow.

I have difficulty parsing the above. The first sentence would suggest
fd_install() tries to detect and recover from the race (it does not),
the next one claims the race needs to be dealt with (it is, by dup2()).

Given that fd_install() does not suffer the burden, this patch removes
the above and instead expands on the race in dup2() commentary.

While here tidy up the docs around fd_install().

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250320102637.1924183-1-mjguzik@gmail.com


Signed-off-by: Christian Brauner <brauner@kernel.org>