Removing swapfile takes a lock for long time

[Paul Goyette <paul%whooppee.com@localhost>]

I've noticed that when you do ``swapctl -d <swapdev>' all accesses to
the count-of-swapped-out-pages are blocked.  Seems that bringing the
out-swapped pages back takes some lock for the entire process, which
can be quite lengthy if there's a lot of swap...

Among other use cases, there's no way to tell how-many-pages-remain-
to-swap-in;  it also seems to prevent running processes from bringing
their own pages back in.

Shouldn't we be occassionally releasing-and-reacquiring the relevant
lock, so that other things can also make progress?


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