f2fs: reuse read_inline_data for f2fs_convert_inline_page

f2fs_convert_inline_page introduce what read_inline_data
already does for copying out the inline data from inode_page.
We can use read_inline_data instead to simplify the code.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>