multipath: fix scsi async tur checker corruption

multipath: fix scsi async tur checker corruption

Since the tur checker runs asynchronously in its own thread, there is nothing
that keeps a path from being orphaned or deleted before the tur thread has
finished. When this happenes the checker struct gets deleted.  However, the tur
thread might still we writing to that memory.  This can lead to memory
corruption.  This patch adds all of the necessary data to the checker context,
and makes the tur thread only use that. This way, if the checker is deleted
while the thread is still using the context, the thread will clean up the
context itself.

Since the context can only be freed when both the thread and the paths checker
structure have stopped needing it, and these can get get finished with the
context asychronously with respect to each other, the context has a holders
counter, protected by a spinlock, to keep track of the users.  When the
counter drops to zero, the context gets freed.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>