lib/nilfs.c: remove broken readonly fs check at nilfs_sync()

nilfs_sync() API function is wrongly checking EROFS error.  It sees
the return value of ioctl instead of errno variable.  This removes the
broken error check from nilfs_sync() instead of fixing it because
EROFS should by rights be used or ignored by callers.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>