f2fs crypto: check CONFIG_F2FS_FS_XATTR for encrypted symlink

Add missed CONFIG_F2FS_FS_XATTR for encrypted symlink inode in order
to avoid unneeded registry of ->{get,set,remove}xattr.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>