OpenZFS 7606 - dmu_objset_find_dp() takes a long time while importing pool

When importing a pool with a large number of filesystems within the same
parent filesystem, we see that dmu_objset_find_dp() takes a long time.
It is called from 3 places: spa_check_logs(), spa_ld_claim_log_blocks(),
and spa_load_verify().

There are several ways to improve performance here:

    1. We don't really need to do spa_check_logs() or
       spa_ld_claim_log_blocks() if the pool was closed cleanly.

    2. spa_load_verify() uses dmu_objset_find_dp() to check that no
       datasets have too long of names.

    3. dmu_objset_find_dp() is slow because it's doing
       zap_value_search() (which is O(N sibling datasets)) to determine
       the name of each dsl_dir when it's opened. In this case we
       actually know the name when we are opening it, so we can provide
       it and avoid the lookup.

This change implements fix #3 from the above list; i.e. make
dmu_objset_find_dp() provide the name of the dataset so that we don't
have to search for it.

Authored by: Matthew Ahrens <[email protected]>
Reviewed by: Steve Gonczi <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Prashanth Sreenivasa <[email protected]>
Reviewed-by: David Quigley <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Ported-by: George Melikov <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/7606
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/cac6bab
Closes #5662 

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