Make /proc/slabinfo use seq_list_xxx helpers

Make /proc/slabinfo use seq_list_xxx helpers

This entry prints a header in .start callback.  This is OK, but the more
elegant solution would be to move this into the .show callback and use
seq_list_start_head() in .start one.

I have left it as is in order to make the patch just switch to new API and
noting more.

[[email protected]: Wrong pointer was used as kmem_cache pointer]
Signed-off-by: Pavel Emelianov <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Signed-off-by: Alexey Dobriyan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>