LU-9344 test: hung with sendfile_grouplock test12()

LU-9344 test: hung with sendfile_grouplock test12()

This is a makeshift fix.

When we hold a group lock of a file, there should no data written to
the file, since during the write IO, the file's layout could possibly
change, and the write IO will try to update its layout, which could
be blocked by itself.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: Ia24310d509e8a93c3c1d849c54eacf789f40bfb8
Reviewed-on: https://review.whamcloud.com/26646
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>