Fix for access beyond end of device error

Fix for access beyond end of device error

This commit fixes a sign extension bug affecting l2arc devices.  Extremely
large offsets may be passed down to the low level block device driver on
reads, generating errors similar to

    attempt to access beyond end of device
    sdbi1: rw=14, want=36028797014862705, limit=125026959

The unwanted sign extension occurrs because the function arc_read_nolock()
stores the offset as a daddr_t, a 32-bit signed int type in the Linux kernel.
This offset is then passed to zio_read_phys() as a uint64_t argument, causing
sign extension for values of 0x80000000 or greater.  To avoid this, we store
the offset in a uint64_t.

This change also changes a few daddr_t struct members to uint64_t in the libspl
headers to avoid similar bugs cropping up in the future.  We also add an ASSERT
to __vdev_disk_physio() to check for invalid offsets.

Closes #66
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>