mzap_upgrade() must use kmem_alloc()

These allocations in mzap_update() used to be kmem_alloc() but
were changed to vmem_alloc() due to the size of the allocation.
However, since it turns out this function may be called in the
context of the txg_sync thread they must be changed back to use
a kmem_alloc() to ensure the KM_PUSHPAGE flag is honored.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>