spl: Add callbacks to invalidate cached devices

spl: Add callbacks to invalidate cached devices

Several SPL functions try to avoid performing initialization twice by
caching devices. This is fine for regular boot, but does not work with
UNIT_TEST, since all devices are torn down after each test. Add some
functions to invalidate the caches which can be called before testing these
load methods.

Signed-off-by: Sean Anderson <seanga2@gmail.com>