unionfs: do not create a new status object during vop_close()

Split the portion of unionfs_get_node_status() that searches for an
existing status object into a new helper function,
unionfs_find_node_status(), and use that in unionfs_close().

Additionally, modify unionfs_close() to accept a NULL status object
if unionfs_find_node_status() does not find a matching status
object. This can happen due to the unconditional VOP_CLOSE()
operation issued by vgonel().

Differential Revision: https://reviews.freebsd.org/D45398
Reviewed by: olce
Tested by: pho