f2fs: relocate inline conversion from mmap() to mkwrite()

If there is page fault only for read case on inline inode, we don't need
to convert inline inode, instead, let's do conversion for write case.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>