LU-13509 ptlrpc: Clear bd_registered in ptlrpc_unregister_bulk

LU-13509 ptlrpc: Clear bd_registered in ptlrpc_unregister_bulk

The patch for LU-12816 https://review.whamcloud.com/36309 has us
clearing the bd_registered flag in ptl_send_rpc(). This flag is set
in ptlrpc_register_bulk(), so it makes sense for us to clear it in
ptlrpc_unregister_bulk(). When we're cleaning up in ptl_send_rpc()
we can be sure the flag is cleared with the call to
ptlrpc_unregister_bulk().

This commit also adds a test case for the LU-12816 bug.

Fixes: e6225c07ce4c ("LU-12816 ptlrpc: ptlrpc_register_bulk LBUG on ENOMEM")
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: Iabaf109aaf72894cd5acbcacbb0299929ea1a146
Reviewed-on: https://review.whamcloud.com/38457
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>