Fix efi_get_info() zvol detection

Partition detection for zvol devices was not working correctly
resulting inconsistent partitioning behavior when layering pools
on top of zvols. This isn't a supported configuration but we'd
still like it to work properly.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7624