LU-10041 osd: osd-zfs to choose dnode size

LU-10041 osd: osd-zfs to choose dnode size

depending on dnodesize property it can be:
legacy (512 bytes), auto (512 bytes to 16K) or absolute
size (512, 1024, 2048, 4096, 8192, 16384).

Change-Id: Iea35d8ae850523440272467320410850821f484c
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/29242
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>