fstests: fix cleanup of test btrfs/003

fstests: fix cleanup of test btrfs/003

If the test fails after removing a device and before adding it back, it
attempts to add back the device in its _cleanup() function. However this
is broken because the device identifier is stored in a variable local to
the function _test_replace() and not in a global variable. So make the
variable global instead of local.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>