BRT: More optimizations after per-vdev splitting

[amotin]

I thought I was done with it, but as often even zillionth code read may give new ideas. ;)

• With both pending and current AVL-trees being per-vdev and having effectively identical comparison functions (pending tree compared also birth time, but I don't believe it is possible for them to be different for the same offset within one transaction group), it makes no sense to move entries from one to another. Instead inline dramatically simplified brt_entry_addref() into brt_pending_apply(). It no longer requires bv_lock, since there is nothing concurrent to it at the time. And it does not need to search the tree for the previous entries, since it is the same tree, we already have the element and we know it is unique.
• Put brt_vdev_lookup() and brt_vdev_addref() into different tree traversals to avoid false positives in the first due to the second entcount modifications. It saves dramatic amount of time when a file cloned first time by not looking for non-existent ZAP entries.
• Remove avl_is_empty(bv_tree) check from brt_maybe_exists(). I don't think it is needed, since by the time all added entries are already accounted in entcount. The extra check must be producing too many false positives for no reason. Also we don't need bv_lock there, since bv_entcount pointer must be stable at this point, and we don't care about false positive races here, while false negative should be impossible, since all brt_vdev_addref() have already completed by this point. This dramatically reduces lock contention on massive deletes of cloned blocks. The only remaining one is between multiple parallel free threads calling brt_entry_decref().
• Do not update ZAP if net change for a block over the TXG was 0. In combination with all above it makes file move between datasets as cheap operation as originally intended if it fits into one TXG.
• Do not allocate vdevs on pool creation or import if it did not have active block cloning. This allows to save a bit in few cases.
• While here, add proper error handling in brt_load() on pool import instead of assertions.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.