xfs: add xlog sysfs kobject and attribute handlers

Embed a kobject into the xfs log data structure (xlog). This creates a
'log' subdirectory for every XFS mount instance in sysfs. The lifecycle
of the log kobject is tied to the lifecycle of the log.

Also define a set of generic attribute handlers associated with the log
kobject in preparation for the addition of attributes.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --cc fs/xfs/xfs_log.c
Simple merge

diff --cc fs/xfs/xfs_log_priv.h
Simple merge

diff --cc fs/xfs/xfs_sysfs.c
Simple merge

diff --cc fs/xfs/xfs_sysfs.h
Simple merge