selinux: drop avtab_search()

avtab_search() shares the same logic with avtab_search_node(), except
that it returns, if found, a pointer to the struct avtab_node member
datum instead of the node itself.  Since the member is an embedded
struct, and not a pointer, the returned value of avtab_search() and
avtab_search_node() will always in unison either be NULL or non-NULL.

Drop avtab_search() and replace its calls by avtab_search_node() to
deduplicate logic and adopt the only caller caring for the type of
the returned value accordingly.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>