f2fs: do not bother checkpoint by f2fs_get_node_info

This patch tries to mitigate lock contention between f2fs_write_checkpoint and
f2fs_get_node_info along with nat_tree_lock.

The idea is, if checkpoint is currently running, other threads that try to grab
nat_tree_lock would be better to wait for checkpoint.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>