UPSTREAM: 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.

Bug: 214413989
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit a9419b63
    git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master)
Change-Id: I7e253e8bf26012e451330ba6ce00bfc3839bdda3