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remove stale comment from swapfile.c
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From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

Seems like on 2.4.9.4 this comment got out of sync ;-)

I'm not completely sure on which basis we don't need any more to do as the
comment suggests, but it seems that when faulting in a second time the same
swap page,  can_share_swap_page() returns false, and we do an early COW break,
so there's no need to write-protect the page.

No idea why we don't defer the COW break.

Reference commit from GIT version of BKCVS history:
5ee46c7964de4b1969fc5be036167eb2da0de4e2, BKRev 3c603c81PtWl2I1NnVuphvsItrD1hg
(v2.4.9.3 -> v2.4.9.4).
