BACKPORT: f2fs: avoid down_write on nat_tree_lock during checkpoint

Let's cache nat entry if there's no lock contention only.

Bug: 214413989
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 0df035c7
    git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master)
[ Address merge conflict ]
Change-Id: I0bf70451bc0b148ebb88429d6294ea8e68008e48