MFC r245264:

MFC r245264:

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)

git-svn-id: svn://svn.freebsd.org/base/stable/9@246574 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f