xfstests: don't remove the two first devices from SCRATCH_DEV_POOL

xfstests: don't remove the two first devices from SCRATCH_DEV_POOL

Since common/config is executed twice, if SCRATCH_DEV_POOL is configured
via the environment, the current code removes the first device entry twice
which means that you lose the second device for the test.

The fix is to not remove anything from SCRATCH_DEV_POOL anymore.
That used to be done (I can only guess) to allow to pass the
SCRATCH_DEV_POOL as an argument to _scratch_mkfs. Since _scratch_mkfs adds
the SCRATCH_DEV, the pool mustn't contain that device anymore.

A new function _scratch_pool_mkfs is introduced that does the expected
thing.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Reviewed-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>