MFC r353176,r353304,r353556,r353559: large_dnode improvements and fixes

MFC r353176,r353304,r353556,r353559: large_dnode improvements and fixes

r353176: MFV r350898, r351075: 8423 8199 7432 Implement large_dnode pool feature
  This updates FreeBSD large_dnode code (that was imported from ZoL) to a
  version that was committed to illumos.  It has some cleanups,
  improvements and fixes comparing to what we have in FreeBSD now.
  I think that the most significant update is 8199 multi-threaded
  dmu_object_alloc().

r353304: zfs: use atomic_load_64 to read atomic variable in dmu_object_alloc_impl

r353556: MFV r353551: 10452 ZoL: merge in large dnode feature fixes

r353559: MFV r353558: 10572 10579 Fix race in dnode_check_slots_free()