f2fs: fix wrong condition to trigger background checkpoint correctly

In f2fs_balance_fs_bg(), it needs to check both NAT_ENTRIES and INO_ENTRIES
memory usage to decide whether we should skip background checkpoint, otherwise
we may always skip checking INO_ENTRIES memory usage, so that INO_ENTRIES may
potentially cause high memory footprint.

Fixes: 493720a4 ("f2fs: fix to avoid REQ_TIME and CP_TIME collision")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>