Fix NULL pointer dereference when doing concurrent 'send' operations

A NULL pointer will occur when doing a 'zfs send -S' on a dataset that
is still being received. The problem is that the new 'send' will
rightfully fail to own the datasets (i.e. dsl_dataset_own_force() will
fail), but then dmu_send() will still do the dsl_dataset_disown().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Luís Henriques <henrix@camandro.org>
Closes #14903
Closes #14890