f2fs: call write_checkpoint under disabled gc

During the write_checkpoint, we should avoid f2fs_gc trigger to avoid any
filesystem consistency.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
f2fs: flush_dcache_page for inline data

When reading inline data, we should call flush_dcache_page.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
f2fs: do not discard data protected by the previous checkpoint

We should not discard any data protected by the previous checkpoint all
the time.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>