f2fs: do not bother checkpoint by f2fs_get_node_info

[ Upstream commit a9419b63 ]

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.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: 3aa51c61 ("f2fs: retry to update the inode page given data corruption")
Signed-off-by: Sasha Levin <sashal@kernel.org>