lib: lmb: rename lmb_get_unreserved_size to lmb_get_free_size

As a follow-up, change the name of the newly introduced function
'lmb_get_unreserved_size' to 'lmb_get_free_size', which is more
appropriate.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
[trini: Fix test/lib/lmb.c]
Signed-off-by: Tom Rini <trini@konsulko.com>