lib: Fix comparison function in link_state.c

lib: Fix comparison function in link_state.c

ls_node_same, ls_attributes_same and ls_prefix_same are not producing expected
result due to a wrong usage of memcmp. In addition, if respective structures
are not initialized with 0, there is a risk that the comparison failed.

This patch correct usage of memcmp and expand comparison to each invidual
parameters of the respective structure for safer result.

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>