ext4: use RCU API in debug_print_tree

struct ext4_sb_info.system_blks was marked __rcu.
But access the pointer without using RCU lock and dereference.
Sparse warning with __rcu notation:

block_validity.c:139:29: warning: incorrect type in argument 1 (different address spaces)
block_validity.c:139:29:    expected struct rb_root const *
block_validity.c:139:29:    got struct rb_root [noderef] <asn:4> *

Link: https://lore.kernel.org/r/20191213153306.30744-1-tranmanphong@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>