gpiosim: fix a resource leak

gpiosim: fix a resource leak

If we set the number of lines to x, set line names or hogs for lines
from 0 through x, then set the number of lines to (x - y), we will not
unlink all the line directories as we only iterate up to num_lines in
bank_release().

Allocate a small structure for every line we create a lineX directory
for and iterate over it in bank_release() to know exactly which ones
need freeing.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>