loader should support large_dnode

[tsoome]

Summary

The zfsonlinux feature large_dnode is not yet supported by the loader.

Test Plan

At the moment the only usable large_dnode feature support is from
zfsonlinux implementation, so the way to test this update is to create
pool and test the reader.

tested with: zfs set dnsize=auto and then did copy some data into dataset, checked which files got dnsize > 512 and verified the loader can read those files.

also zfs set dnsize=1k, did use setfattr to set value < 512 and value > 1k, to verify file without and with spill blocks. The pool and dataset setup was done with zol rescue ISO, read test with loader.efi.

[tsoome]

svn update

[avg]

LGTM

[allanjude]

Thanks for keeping on top of this