LU-5772 osd-zfs: irrelevant comment over __osd_xattr_load

LU-5772 osd-zfs: irrelevant comment over __osd_xattr_load

Moved comment over __osd_xattr_load() to __osd_xattr_get(),
where it really belongs, and converted it to Doxygen format.
Added a few other minor cleanups as well.

Signed-off-by: Isaac Huang <he.huang@intel.com>
Change-Id: If73048d046419eaa4e23cbc5acde32c09b588996
Reviewed-on: http://review.whamcloud.com/12350
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>