Fix panic mounting unformatted zvol

On some older kernels, i.e. 2.6.18, zvol_ioctl_by_inode() may get passed a NULL
file pointer if the user tries to mount a zvol without a filesystem on it.
This change adds checks to prevent a null pointer dereference.

Closes #73.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>