[PATCH] swap: update swapfile i_sem comment

Update swap extents comment: nowadays we guard with S_SWAPFILE not i_sem.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>