rcutorture: Permit kvm-again.sh to re-use the build directory

rcutorture: Permit kvm-again.sh to re-use the build directory

This commit adds "inplace" and "inplace-force" values to the kvm-again.sh
"--link" argument, which causes the run's output to be placed into the
build directory.  This could be used to save build time if the machine
went down partway into a run, but it can also be used to do a large
number of builds, and run the resulting kernels concurrently even if the
builds are based on different commits.  A later commit will add this
latter capability to kvm-series.sh in order to produce large speedups
for branch-checking operations.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>