f2fs: enable fast symlink by utilizing inline data

Fast symlink can utilize inline data flow to avoid using any
i_addr region, since we need to handle many cases such as
truncation, roll-forward recovery, and fsck/dump tools.

Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>