staging: lustre: mgc: handle config_llog_data::cld_refcount properly

Originally, the logic of handling config_llog_data::cld_refcount
is some confusing, it may cause the cld_refcount to be leaked or
trigger "LASSERT(atomic_read(&cld->cld_refcount) > 0);" when put
the reference. This patch clean related logic as following:

1) When the 'cld' is created, its reference is set as 1.

2) No need additional reference when add the 'cld' into the list
   'config_llog_list'.

3) Inrease 'cld_refcount' when set lock data after mgc_enqueue()
   done successfully by mgc_process_log().

4) When mgc_requeue_thread() traversals the 'config_llog_list',
   it needs to take additional reference on each 'cld' to avoid
   being freed during subsequent processing. The reference also
   prevents the 'cld' to be dropped from the 'config_llog_list',
   then the mgc_requeue_thread() can safely locate next 'cld',
   and then decrease the 'cld_refcount' for previous one.

5) mgc_blocking_ast() will drop the reference of 'cld_refcount'
   that is taken in mgc_process_log().

6) The others need to call config_log_find() to find the 'cld'
   if want to access related config log data. That will increase
   the 'cld_refcount' to avoid being freed during accessing. The
   sponsor needs to call config_log_put() after using the 'cld'.

7) Other confused or redundant logic are dropped.

   On the other hand, the patch also enhances the protection for
   'config_llog_data' flags, such as 'cld_stopping'/'cld_lostlock'
   as following.

a) Use 'config_list_lock' (spinlock) to handle the possible
   parallel accessing of these flags among mgc_requeue_thread()
   and others config llog data visitors, such as mount/umount,
   blocking_ast, and so on.

b) Use 'config_llog_data::cld_lock' (mutex) to pretect other
   parallel accessing of these flags among kinds of blockable
   operations, such as mount, umount, and blocking ast.

The 'config_llog_data::cld_lock' is also used for protecting
the sub-cld members, such as 'cld_sptlrpc'/'cld_params', and
so on.
Signed-off-by: Fan Yong <fan.yong@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8408
Reviewed-on: http://review.whamcloud.com/21616Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>