BACKPORT: 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.

Bug: 208371174
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 58c3b33eb9f1)
Change-Id: I68168bc08a77d8d8f039189d44d23f83a7eda1f5