OK, upgrading to gentoo-sources 4.1.11 didn't help, so I tried these steps.  
More inline below.

On Tuesday 27 October 2015 06:23:06 Duncan wrote:
>Marc Joliet posted on Mon, 26 Oct 2015 15:23:39 +0100 as excerpted:
>> Occasionally they go away by themselves, but usually I have to reboot to
>> make them go away.  This happens when getmail attempts to fetch mail,
>> which fails due to the above error.  After the reboot getmail succeeds
>> again.
>
>Just out of curiosity, does a remount,ro, followed by a remount,rw, solve
>the problem?
>
>The ro/rw cycle should force anything in memory to device, so if that
>eliminates the problem, it could well be some sort of sync issue.  If it
>doesn't, then it's more likely an in-memory filesystem state issue,
>that's cleared by the reboot.

Didn't try this directly, but...

>And if the ro/rw cycle doesn't clear the problem, what about a full
>unmount/mount cycle, at least of that subvolume?

...this didn't work, after which...

>If you're running multiple subvolumes with root being one of them, you
>can't of course unmount the entire filesystem, but you could go down to
>emergency mode (systemctl emergency), try unmounting everything but /,
>mounting / ro, and then switching back to normal mode (from emergency
>mode, simply exiting should return you to normal multi-user or gui
>target, remounting filesystems as necessary, etc).

...I tried most of this.  I unmounted everything except for /var and /, 
neither of which I could mount read-only.  It didn't help, either.

>IOW, does it take a full reboot to clear the problem, or is a simple ro/rw
>mount cycle enough, or an unmount/remount?

Seems that a full reboot is needed, but I would expect that it would have the 
same effect if I were to pivot back into the initramfs, unmount / from there, 
then boot back into the system.  Because quite frankly, I can't think of any 
reason why a power cycle to the SSD should make a difference here.  I vaguely 
remember that systemd can do that, so I'll see if I can find out how.

>Finally, assuming root itself isn't btrfs, if you have btrfs configured
>as a module, you could try unmounting all btrfs and then unloading the
>module, then reloading and remounting.  That should entirely clear all in-
>memory btrfs state, so if that doesn't solve the problem, while rebooting
>does, then the problem's very possibly outside of btrfs scope.  Of course
>if root is btrfs, you can't really check that.

Nope, btrfs is built-in (though it doesn't have to be, what with me using an 
initramfs).

Thanks
-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup