locks: fix file_lock deletion inside loop

locks_delete_lock_ctx() is called inside the loop, so we
should use list_for_each_entry_safe.

Fixes: 8634b51f (locks: convert lease handling to file_lock_context)
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>