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

Change-Id: If099f83973825981b4c568db7572bf30d399c089
Reviewed-on: http://gerrit.openafs.org/10251
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>