posix-timers: Protect posix clock array access against speculation

The clockid argument of clockid_to_kclock() comes straight from user space
via various syscalls and is used as index into the posix_clocks array.

Protect it against spectre v1 array out of bounds speculation. Remove the
redundant check for !posix_clock[id] as this is another source for
speculation and does not provide any advantage over the return
posix_clock[id] path which returns NULL in that case anyway.

Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Dan Williams <[email protected]>
Cc: Rasmus Villemoes <[email protected]>
Cc: Greg KH <[email protected]>
Cc: [email protected]
Cc: Linus Torvalds <[email protected]>
Cc: David Woodhouse <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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