common/rc: create a wrapper for the su command

common/rc: create a wrapper for the su command

Create a _su wrapper around the su command so that the next patch can
fix all the pkill isolation code so that this test can only kill
processes started for this test.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>