Pulling to netbsd-9 branch fixes for #54977

[tlaronde%polynum.com@localhost]

Hello,

I have experienced a USB failure with an excessive amount of file cache,
while the mounted filesystems shouldn't have this lot of blocks in
cache: this was likely due to a rsync(1) failure on an USB connected
disk. The USB was detached ("file system full") while rsync(1) was
operating but the files stayed in cache and the umass0 was not
reattachable when trying a "drvctl -r umass0".

Very likely PR #54977 (in my case: an ARM SoC with only 1GB of memory).

There are fixes in current (mentionned in PR).

Could they be pulled to the 9 branch?

Best,
-- 
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
                     http://www.kergis.com/
                    http://kertex.kergis.com/
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