Fix ZFS bug before releasing 10.3

[Martin Birgmeier]

Hi,

Could someone with enough knowledge please look into
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207714 and fix it
before 10.3?

It is rather nasty as it corrupts files with holes when doing 'zfs
send/zfs receive'. Such files appear 'automatically' if compression is
enabled and a large enough chunk of zeros exists in a file.

Pointer to a possible patch is attached in the PR.

-- Martin