Fix dnode_hold_impl() soft lockup

Soft lockups could happen when multiple threads trying
to get zrl on the same dnode handle in order to allocate
and initialize the dnode marked as DN_SLOT_ALLOCATED.

Don't loop from beginning when we can't get zrl, otherwise
we would increase the zrl refcount and nobody can actually
lock it.

Reviewed by: Tom Caputi <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Li Dongyang <[email protected]>
Closes #8433 

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