firmware: arm_sdei: Document the motivation behind these set_fs() calls

The SDEI handler save/restores the addr_limit using set_fs(). It isn't
very clear why. The reason is to mirror the arch code's entry assembly.
The arch code does this because perf may access user-space, and
inheriting the addr_limit may be a problem.

Add a comment explaining why this is here.

Suggested-by: Christoph Hellwig <[email protected]>
Signed-off-by: James Morse <[email protected]>
Link: https://bugs.chromium.org/p/project-zero/issues/detail?id=822
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

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