- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

The current ZFS code expects ddt_zap_count to always succeed by asserting
the underlying zap_count() to return no errors. However, it is possible
that the pool reaches to such a state where zap_count would return error,
leading to panics when a pool is imported.

This commit changes the ddt_zap_count to return error returned from
zap_count and handle the error appropriately. With this change, it's now
possible to let zpool rollback damaged transaction groups and import the
pool.

Obtained from: ZFS on Linux github (e8fd45a0f975c6b8ae8cd644714fc21f14fac2bf)
MFC after: 1 month

Finally... Import the latest open-source ZFS version - (SPA) 28.

Few new things available from now on:

- Data deduplication.
- Triple parity RAIDZ (RAIDZ3).
- zfs diff.
- zpool split.
- Snapshot holds.
- zpool import -F. Allows to rewind corrupted pool to earlier
transaction group.
- Possibility to import pool in read-only mode.

MFC after: 1 month