building netbsd-9 2 'sync' processes stuck in 'tstile'

["John D. Baker" <jdbaker%consolidated.net@localhost>]

While building netbsd-9/amd64 with "-j 2", the build process got stuck
while linking "GENERIC_KASLR" and "GENERIC".  'top' shows two 'sync'
processes stuck in 'tstile'.  Although the build could be aborted with
"Ctrl-C", the two 'sync' processes remain and cannot be killed (even
with -9).

The host is netbsd-9/amd64 as of 30 April.  The filesystem on which the
build process operates resides on a local RAIDframe RAID-R of eight 1TB
SATA disks.

The same filesystem is also NFS exported and clients otherwise continue
to operate on it normally.

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