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()