linux: core dump requires write fop

turns out not just writev is unhappy with aio_write (only); core dumping
wants a write file op. always provide it.

FIXES 131729

Reviewed-on: http://gerrit.openafs.org/10251
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 13165c05836bcd4b3f5655ab2bcc979ff859efa2)

Change-Id: I35f16004164af79621307de1973d25e75b01ec2c
Reviewed-on: http://gerrit.openafs.org/10254
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>