swapfile: swapon use discard (trim)

When adding swap, all the old data on swap can be forgotten: sys_swapon()
discard all but the header page of the swap partition (or every extent but
the header of the swap file), to give a solidstate swap device the
opportunity to optimize its wear-levelling.

If that succeeds, note SWP_DISCARDABLE for later use, and report it with a
"D" at the right end of the kernel's "Adding ...  swap" message.  Perhaps
something should be shown in /proc/swaps (swapon -s), but we have to be
more cautious before making any addition to that format.

Signed-off-by: Hugh Dickins <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Joern Engel <[email protected]>
Cc: James Bottomley <[email protected]>
Cc: Donjun Shin <[email protected]>
Cc: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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