f2fs: fix to update upper_p in __get_secs_required() correctly

Commit 1acd73ed ("f2fs: fix to account dirty data in __get_secs_required()")
missed to calculate upper_p w/ data_secs, fix it.

Fixes: 1acd73ed ("f2fs: fix to account dirty data in __get_secs_required()")
Cc: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>