f2fs: fix to use per-inode maxbytes and cleanup

This is a supplement to commit 6d1451bf ("f2fs: fix to use per-inode maxbytes")
for some missed cases, also cleanup redundant code in f2fs_llseek.

Cc: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>