zvol_probe should return NULL when the device isn't found.

Previously we returned ERR_PTR(-ENOENT) which the rest of the kernel
doesn't expect and as such we can oops.

Signed-off-by: Chris Wedgwood <cw@f00f.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #949
Closes #931
Closes #789
Closes #743
Closes #730