Verify embedded blkptr's in arc_read()

The block pointer verification check in arc_read() should also
cover embedded block pointers. While highly unlikely, accessing
a damaged block pointer can result in panic. To further harden
the code extend the existing check to include embedded block
pointers and add a comment explaining the rational for this
sanity check. Lastly, correct a flaw in zfs_blkptr_verify()
so the error count is checked even when checking a untrusted
config to verify the non-pool-specific portions of a block
pointer.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12535