xfstests: Change the interpreter of ltp/runtest.sh to bash

xfstests: Change the interpreter of ltp/runtest.sh to bash

In some environment, Running 080 fails with many errors.

It happens because ltp/rwtest.sh assumes /bin/sh is `bash' and it's not
always true (ex. /bin/sh is `dash' in Debian.) This script should
explicitly declare that its interpreter is `bash'.

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>