MFC r253821,254753,256259

MFV r253783: 3834 incremental replication of 'holey' file systems is
slow

MFV r254747:4047 panic from dbuf_free_range() from dmu_free_object()
while doing zfs receive

MFV r255257: 4082 zfs receive gets EFBIG from dmu_tx_hold_free()
MFC r258354: taskqueue_cancel: garbage collect a write-only variable
MFC r258351: fsx: new option to disable msync(MS_SYNC) after each write
MFC r258352: fsx: add an option to randomly call msync(MS_INVALIDATE)
MFC r258638,258642: expose zfs_flags as debug.zfs_flags r/w tunable and sysctl

Sponsored by:	HybridCluster
MFC r258647: zfsboottest: properly specify a library dependency