It looks like you're trying to run fsck on a read/write filesystem. You might try booting into single-user mode, passing the kernel command-line “ro s” (ro for read-only, s for single-user). You should then be able to check the filesystem manually using fsck.
Failing that, you will need to boot from a rescue disk (so that the filesystem's completely unused) and run fsck from there.
Hope that helps,
— Robert J. Lee