linux: fsync on a directory should return 0, not EINVAL

linux: fsync on a directory should return 0, not EINVAL

Directory writes are synchronous, so this is fine. There's a
mostly-convenient function in fs/libfs.c that returns 0 that we can use
to do what we want ("mostly" because it was renamed in 2.6.35).

FIXES 130425

Change-Id: I9a2af60ed3152be036f0145c94152d8cff2e1242
Reviewed-on: http://gerrit.openafs.org/6491
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>