[CIFS] Use the kthread_ API instead of opencoding lots of hairy code for kernel

thread creation and teardown.

It does not move the cifsd thread handling to kthread due to problems
found in testing with wakeup of threads blocked in the socket peek api,
but the other cifs kernel threads now use kthread.
Also cleanup cifs_init to properly unwind when thread creation fails.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>