swapon before closing trimming fd

[dougm]

Summary

To make swapon -E work with geli detach-on-last-close, don't close the trimming fd until after swapon(2) is invoked.

Test Plan

Ask reviewers to confirm that it fixes geli, and doesn't break swapon trimming for anyone else.

[sigsys_gmail.com]

Probably not safe to start trimming the swap device *after* the kernel has been told to start using it for swapping (even if the window is probably small).

[dougm]

Call swapon(2) after trimming, but before closing the trimming fd.

[sigsys_gmail.com]

In D21006#455769, @dougm wrote:

Call swapon(2) after trimming, but before closing the trimming fd.

Works here. This is cleaner than what I was doing for sure. Got a typo though, it's "Eflag" not "Eflags".

[dougm]

Fix Eflags typo.

[markj]

sbin/swapon/swapon.c

765 ↗

(On Diff #59960)

I think a comment explaining why this function handles both trimming and swapon would be appropriate.

[dougm]

Add explanatory comment.