Allow sbld allocation to fail when PMAP_CANFAIL.

Allow sbld allocation to fail when PMAP_CANFAIL.

pool_get in powerpc64's pmap might fail when uvm_km_pages.free == 0.
This might cause KASSERT(slbd) to panic 'kernel diagnostic assertion
"sbld" failed'.  Remove this KASSERT and add a check for PMAP_CANFAIL.
pmap_enter calls without PMAP_CANFAIL might still panic.

ok mpi@ jca@