mm/cma_debug.c: remove static scoped cma_debugfs_root

mm/cma_debug.c: remove static scoped cma_debugfs_root

Currently cma_debugfs_root is static storage.  That is unnecessary since
it will be only used by next cma_debugfs_add_one().  We can just pass it
to following calling to save thisspace.  Also remove useless idx
parameter.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yue Hu <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joe Perches <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>