MFV r243013 and r243267:

MFV r243013 and r243267:

Import the zio nop-write improvement from Illumos. To reduce I/O,
nop-write omits overwriting data if the checksum (cryptographically
secure) of new data matches the checksum of existing data.
It also saves space if snapshots are in use.

It currently works only on datasets with enabled compression, disabled
deduplication and sha256 checksums.

IllumOS 13887:196932ec9e6a and 13888:7204b3392a58
3236 zio nop-write

References:
https://www.illumos.org/issues/3236

MFC after:	2 weeks